[ci] Use variables to control system .NET SDK installs

[pjcollins]

I noticed the net10.0 branch build was failing during the
"generate and publish BAR manifest" step with:

D:\a\_work\1\s\dotnet\package\common.csproj(151,5): error MSB4062: The "PushToBuildStorage" task could not be loaded from the assembly D:\a\_work\1\s\packages\microsoft.dotnet.build.tasks.feed\10.0.0-beta.25228.101\build\../tools/net/Microsoft.DotNet.Build.Tasks.Feed.dll. Could not load file or assembly 'System.Runtime, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. The system cannot find the file specified.

The Microsoft.DotNet.Build.Tasks.Feed tasks are seemingly now being
built against .NET 10, so we'll need to install .NET 10 to run them.

Variables that represent the current stable and preview versions of .NET
have been added to be used throughout the pipeline. These will now only

[Copilot]

Pull Request Overview

This PR updates various CI pipeline templates to use variables for specifying .NET SDK versions, allowing centralized control over stable and preview .NET SDK installations.

• Replaces hard-coded SDK versions (e.g., “9.x”) with variable references in tasks such as UseDotNet@2.
• Adds variables in the common variables file for stable (9.0) and preview (10.0) .NET SDK versions and adjusts branch matching accordingly.
• Updates display names and pipeline branch conditions to reflect the new variable-driven approach.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tools/devops/automation/templates/windows/reserve-mac.yml	Updated UseDotNet task to use $(DotNetSdkVersion)
tools/devops/automation/templates/windows/build.yml	Updated UseDotNet task to use $(DotNetSdkVersion)
tools/devops/automation/templates/variables/common.yml	Introduced DotNetSdkVersion (stable) and DotNetPreviewSdkVersion (preview) variables
tools/devops/automation/templates/tests/build.yml	Updated UseDotNet task to use $(DotNetSdkVersion)
tools/devops/automation/templates/release/vs-insertion-prep.yml	Updated task displayName and SDK version input for preview installs
tools/devops/automation/templates/main-stage.yml	Replaced explicit branch names with variable-based names for SDK versions
tools/devops/automation/templates/mac/build.yml	Updated UseDotNet task to use $(DotNetSdkVersion)
eng/common/core-templates/job/source-index-stage1.yml	Updated displayName and SDK version input from hard-coded 8.x to variable-driven version

Comments suppressed due to low confidence (2)

eng/common/core-templates/job/source-index-stage1.yml:52

• The display name has changed from 'Use .NET 8 SDK' to use the variable $(DotNetSdkVersion) (currently 9.0). Please verify this change was intentional and that all dependencies relying on .NET 8 are updated accordingly.

displayName: Use .NET $(DotNetSdkVersion) SDK

tools/devops/automation/templates/main-stage.yml:118

• Using variable substitution in branch comparison strings may not resolve as expected. Please confirm that the resulting branch names (e.g., 'refs/heads/net9.0') match the intended branch naming conventions.

eq(variables['Build.SourceBranch'], 'refs/heads/net$(DotNetSdkVersion)')

[pjcollins]

@rolfbjarne this should fix the .NET 10 builds and allow #22660 to build completely again. I figured it would be best to put it on main first, hopefully CI cooperates.

[rolfbjarne]

Do we need the .NET 10 preview in the stable .NET 9 branch? If not, I think this should be 9.0 in .NET 9 branch(es) and then bumped to .NET 10 in net10.0

[pjcollins]

Nothing explicitly needs it. I kept this at 10.0 so that it can be used in the list of branch triggers in main-stage.yml,
and updated vs-insertion-prep.yml to continue to use the stable version on main instead.

[pjcollins]

This didn't apply cleanly to net10.0 so I've put up a PR for that branch here #22704

[vs-mobiletools-engineering-service2]

✅ [CI Build #3443e0e] Build passed (Build packages) ✅

Pipeline on Agent
Hash: 3443e0e0732857a0f2a709dd0c37421ccbaa52e5 [PR build]

[vs-mobiletools-engineering-service2]

✅ [CI Build #3443e0e] Build passed (Detect API changes) ✅

Pipeline on Agent
Hash: 3443e0e0732857a0f2a709dd0c37421ccbaa52e5 [PR build]

[vs-mobiletools-engineering-service2]

✅ API diff for current PR / commit

.NET ( No breaking changes )

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

✅ API diff vs stable

.NET ( No breaking changes )

• iOS: vsdrops gist ( No breaking changes )
• tvOS: vsdrops gist ( No breaking changes )
• MacCatalyst: vsdrops gist ( No breaking changes )
• macOS: vsdrops gist ( No breaking changes )

✅ Generator diff

Generator diff is empty

Pipeline on Agent
Hash: 3443e0e0732857a0f2a709dd0c37421ccbaa52e5 [PR build]

[vs-mobiletools-engineering-service2]

✅ [CI Build #0ccb7c9] Build passed (Build macOS tests) ✅

Pipeline on Agent
Hash: 0ccb7c980db83e1b4591418aae42844e2dc5d26d [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build #0ccb7c9] Tests on macOS X64 - Mac Sonoma (14) passed 💻

✅ All tests on macOS X64 - Mac Sonoma (14) passed.

Pipeline on Agent
Hash: 0ccb7c980db83e1b4591418aae42844e2dc5d26d [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build #0ccb7c9] Tests on macOS M1 - Mac Monterey (12) passed 💻

✅ All tests on macOS M1 - Mac Monterey (12) passed.

Pipeline on Agent
Hash: 0ccb7c980db83e1b4591418aae42844e2dc5d26d [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build #0ccb7c9] Tests on macOS M1 - Mac Ventura (13) passed 💻

✅ All tests on macOS M1 - Mac Ventura (13) passed.

Pipeline on Agent
Hash: 0ccb7c980db83e1b4591418aae42844e2dc5d26d [PR build]

[vs-mobiletools-engineering-service2]

💻 [CI Build #0ccb7c9] Tests on macOS arm64 - Mac Sequoia (15) passed 💻

✅ All tests on macOS arm64 - Mac Sequoia (15) passed.

Pipeline on Agent
Hash: 0ccb7c980db83e1b4591418aae42844e2dc5d26d [PR build]