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

…ity (#60881)

* Disable package validation in source-build for reliability

* Add tracking issue to comment

* Switch from property overwrite to a condition

eng/packaging.targets

@@ -1,7 +1,13 @@
 <Project>
 
   <PropertyGroup>
-    <EnablePackageValidation>true</EnablePackageValidation>
+    <!--
+      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.
+    -->
+    <EnablePackageValidation Condition="'$(DotNetBuildFromSource)' != 'true'">true</EnablePackageValidation>
     <!-- Don't restore prebuilt packages during sourcebuild. -->
     <DisablePackageBaselineValidation Condition="'$(DotNetBuildFromSource)' == 'true'">true</DisablePackageBaselineValidation>
     <PackageValidationBaselineVersion Condition="'$(PackageValidationBaselineVersion)' == ''">$([MSBuild]::Subtract($(MajorVersion), 1)).0.0</PackageValidationBaselineVersion>