SDK main uses dotnetup to install the bootstrap SDK and test runtimes

[nagilson]

For #54257

Port the dotnetup-based acquisition scripts from https://github.com/dotnet/sdk/tree/release/dnup.
The implementation is reviewed within #54586. No code is new within ce484c3, but I could not reasonably cherry-pick, the commits were too sharded. History will continue to be preserved in release/dnup.

In this PR, we:
configure-toolset installs the bootstrap SDK from global.json and restore-toolset installs the test shared frameworks, both via dotnetup (downloaded by scripts/get-dotnetup.{sh,ps1}).
The dotnet-install script is retained as a fallback.

Modify .gitignore to Ignore the downloaded eng/dotnetup/ binary.

[Copilot]

Pull request overview

This PR ports the dotnetup-based toolset acquisition flow into main, using dotnetup to install the bootstrap SDK from global.json and to install shared frameworks needed for tests, with dotnet-install retained as a fallback.

Changes:

• Add scripts/get-dotnetup.{sh,ps1} to download and verify dotnetup binaries via aka.ms + SHA-512.
• Update eng/configure-toolset.{sh,ps1} to install the bootstrap SDK via dotnetup into the repo-local SDK location.
• Update eng/restore-toolset.{sh,ps1} to install test runtimes via dotnetup with fallback behavior for cross-arch scenarios, and ignore the cached dotnetup binary directory.

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
scripts/get-dotnetup.sh	Adds a Bash helper to download, checksum-verify, and install dotnetup locally.
scripts/get-dotnetup.ps1	Adds a PowerShell helper to download, checksum-verify, and install dotnetup locally.
eng/restore-toolset.sh	Switches test runtime acquisition to dotnetup with install-script fallback when arch override is needed.
eng/restore-toolset.ps1	Switches test runtime acquisition to dotnetup with install-script fallback when arch override is needed.
eng/configure-toolset.sh	Pre-installs bootstrap SDK(s) via dotnetup as part of toolset configuration.
eng/configure-toolset.ps1	Pre-installs bootstrap SDK(s) via dotnetup as part of toolset configuration.
.gitignore	Ignores the downloaded eng/dotnetup/ binary directory.

[nagilson]

I will force merge upon approval, similar to how I force merged here, #54586 to unblock. I verified the failures were due to flakey tests -

this was due to a .cpp test which is known to cause issues and not related to the dotnetup changes. All legs were building well with dotnetup.

[dsplaisted]

As discussed, let's follow up after this by switching to live versions of the get-dotnetup scripts from the aka.ms links.

[nagilson]

As discussed, let's follow up after this by switching to live versions of the get-dotnetup scripts from the aka.ms links.

Roger that: #54606 (working on it)