Use architecture-appropriate desktop MSBuild and remove Prefer64bit switch

[Copilot]

Desktop MSBuild was always launched as x86, even on amd64/arm64 hosts. The Prefer64bit opt-in (from #7531) never became the default, so the x64 MSBuild was effectively unreachable, and arm64 was never considered.

GetMSBuildExe in eng/common/tools.ps1 now selects the desktop MSBuild matching the host process architecture:

• Architecture detection — reads PROCESSOR_ARCHITECTURE, falling back to PROCESSOR_ARCHITEW6432 so a 32-bit process on a 64-bit OS resolves the real machine arch.
• Subfolder mapping — AMD64 → amd64\msbuild.exe, ARM64 → arm64\msbuild.exe; anything else uses the root (32-bit) msbuild.exe.
• Safe fallback — if the arch-specific subfolder isn't present, falls back to the root msbuild.exe.
• Removed the Prefer64bit switch and its read from global.json (tools.vs), since arch-appropriate selection is now unconditional.

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[Copilot]

Pull request overview

Updates Arcade’s Visual Studio MSBuild discovery to select a desktop msbuild.exe that matches the effective host architecture (including WOW64 scenarios), removing the previous Prefer64bit opt-in path that made non-x86 MSBuild effectively unreachable in many cases.

Changes:

• Update InitializeVisualStudioMSBuild to choose Bin\amd64\msbuild.exe or Bin\arm64\msbuild.exe based on host/process environment architecture, with a safe fallback to Bin\msbuild.exe.
• Remove the Prefer64bit selection logic.

Show a summary per file

File	Description
eng/common/tools.ps1	Updates MSBuild exe selection to be architecture-appropriate by default and removes the Prefer64bit switch logic.

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