Fix Unix build bootstrap runtime resolution for test apphosts

[Copilot]

Bug Fix

The build/test pipeline could launch test apphosts against machine-installed runtimes instead of the repo-bootstrapped .dotnet, causing runtime mismatch failures and cascading broken tests. This change aligns Unix bootstrap behavior with expected runtime resolution during build-driven test execution.

• What was the bug?

  • eng/common/tools.sh prepended the repo-local dotnet to PATH, but did not export DOTNET_ROOT.
  • Test apphosts executed during build/test/integration flows could resolve runtime from system locations (for example /usr/share/dotnet) rather than the repo toolset.

• How did you fix it?

  • Export DOTNET_ROOT from the resolved bootstrap root ($dotnet_root) during Unix toolset initialization.
  • Publish the same value via pipeline variable emission to keep child process/environment behavior consistent.

# eng/common/tools.sh
Write-PipelinePrependPath -path "$dotnet_root"
export DOTNET_ROOT="$dotnet_root"

Write-PipelineSetVariable -name "DOTNET_ROOT" -value "$dotnet_root"

• Scope of change

  • Single-file, bootstrap-only update in eng/common/tools.sh.
  • No product/runtime logic changes; only environment wiring for build/test process execution.

Original prompt

Fix build issues and broken tests

The user has attached the following file paths as relevant context:

• .github\copilot-instructions.md

Created from VS Code.

[Copilot]

Pull request overview

This PR fixes Unix build/test runtime resolution by ensuring apphosts and child processes consistently use the repo-bootstrapped .dotnet rather than a machine-installed runtime.

Changes:

• Export DOTNET_ROOT to the resolved $dotnet_root during Unix toolset initialization.
• Emit DOTNET_ROOT as a pipeline variable to keep CI child process environments aligned with the bootstrap SDK.

Show a summary per file

File	Description
eng/common/tools.sh	Exports and publishes DOTNET_ROOT so runtime/apphost resolution uses the bootstrapped SDK consistently.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0