[release/8.0] Update APICompat settings under source build (#93865)

* Update APICompat settings under source build

* Update resolveContract.targets

---------

Co-authored-by: Viktor Hofer <viktor.hofer@microsoft.com>

eng/resolveContract.targets

@@ -73,8 +73,9 @@
        That is necessary as APICompat is invoked twice, once for the ref <-> src comparision and then again
        for the package validation (which doesn't include reference assemblies). As both operations don't have
        all the inputs available, some suppressions might only apply to one or the other and hence unnecessary
-       suppressions can't be determined. -->
-  <PropertyGroup Condition="'$(IsPackable)' == 'true' and '$(IsRuntimeAndReferenceAssembly)' != 'true'">
+       suppressions can't be determined.
+       Disable the validation under source build as that might use an out-of-date SDK and not the ApiCompat.Task package. -->
+  <PropertyGroup Condition="('$(IsPackable)' == 'true' and '$(IsRuntimeAndReferenceAssembly)' != 'true') or '$(DotNetBuildFromSource)' == 'true'">
     <ApiCompatPreserveUnnecessarySuppressions>true</ApiCompatPreserveUnnecessarySuppressions>
     <ApiCompatPermitUnnecessarySuppressions>true</ApiCompatPermitUnnecessarySuppressions>
   </PropertyGroup>