Fix ImageBuilder test toolset resolution

[lbussell]

This PR fixes docker-tools-imagebuilder-official failing in Test Images after the Arcade update in #2099.

The root cause is that Arcade stopped writing artifacts/toolset/<version>.txt and now restores the SDK toolset to artifacts/toolset/<version>/Build.proj.

Relevant Arcade history:

• [main] Source code updates from dotnet/dotnet arcade#16726 originally introduced the InitializeToolset layout change.
• Revert "[main] Source code updates from dotnet/dotnet" arcade#16744 reverted that source update.
• [main] Source code updates from dotnet/dotnet arcade#16753 reintroduced the layout change; this is the change present in the Arcade build consumed by [main] Update dependencies from dotnet/arcade #2099.

This PR updates src/run-tests.ps1 to resolve the new Build.proj location first while retaining a legacy marker-file fallback for older Arcade layouts.

Validation:

• git clean -ffxd
• pwsh -NoLogo -NoProfile -NonInteractive -File ./src/run-tests.ps1 (478 tests passed)

Fixes #2104