[release/6.0] Disable package validation in source-build for reliability

[dagood]

  Package validation causes flaky errors during the build. Validation isn't useful during
  source-build (it is only a guardrail for devs), so disable it during source-build to avoid the
  impact on build reliability.

• For 6.0 dotnet/runtime: Microsoft.DotNet.Compatibility.ValidatePackage fails, "System.MissingMethodException: Method not found: '...'" source-build#2501

This has been fixed in main, but not release/6.0. I'm submitting this PR to fix 6.0 source-build reliability in the short term while waiting for the real fixes to be ported to 6.0:

• Update compatibility package and unpin compiler version #60847
• Improve loading of Roslyn assemblies on validate package task sdk#22277

For followup removal:

• https://github.com/dotnet/runtime/issues/60883

@safern

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Issue Details

---

  Package validation causes flaky errors during the build. Validation isn't useful during
  source-build (it is only a guardrail for devs), so disable it during source-build to avoid the
  impact on build reliability.

• For 6.0 dotnet/runtime: Microsoft.DotNet.Compatibility.ValidatePackage fails, "System.MissingMethodException: Method not found: '...'" source-build#2501

This has been fixed in main, but not release/6.0. I'm submitting this PR to fix 6.0 source-build reliability in the short term while waiting for the real fixes to be ported to 6.0:

• Update compatibility package and unpin compiler version #60847
• Improve loading of Roslyn assemblies on validate package task sdk#22277

@safern

Author:	dagood
Assignees:	-
Labels:	area-Infrastructure-libraries
Milestone:	-