Disable package validation in source-build for reliability

[lbussell]

See dotnet/source-build#2708: The source-build team is making an effort to upstream all of our patches over the next week.

This is a port of #60881 to main. Description from the comment in the diff:

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. https://github.com/dotnet/runtime/issues/60883 tracks fixing the
flakiness and removing this condition.

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Issue Details

---

This is a port of #60881 to main. Description from the comment in the diff:

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. https://github.com/dotnet/runtime/issues/60883 tracks fixing the
flakiness and removing this condition.

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

[safern]

@lbussell just curious, why is this needed in main? We fixed the reliability issues we were hitting on PackageValidation on main already and we haven't seen this error in months on the dotnet/runtime main branch.

[lbussell]

just curious, why is this needed in main? We fixed the reliability issues

Oh, I see, I didn't catch that the reliability issue has been fixed in main. We're still carrying a patch for this change in source build on installer/main, so I assumed this change was necessary. I'll give a shot at deleting the patch and if it's reliable, I'll close this PR.

[safern]

Thanks!