Only copy restored packages for source only builds

[mthalman]

This issue stems from my comment at dotnet/installer#18824 (comment).

The CopyInnerBuildRestoredPackages task copies restored packages to the .packages directory at the repo root. AFAICT, this collection of packages is used as a means for prebuilt detection, which is done by the ReportPrebuiltUsage target. The issue is that this also gets executed for the unified-build where prebuilt detection isn't relevant. So in the unified-build it's wasting time and disk space for no purpose.

This should get cleaned up so it's only done for source builds. In addition, the ReportPrebuiltUsage target should get moved into the same file from where it's referenced.

[mthalman]

Fixed by dotnet/installer#19072

In addition, the ReportPrebuiltUsage target should get moved into the same file from where it's referenced.

This can't be done because build.sourcebuild.targets isn't imported by repo-projects.