Source-build release pipeline should collect artifacts from PreviousSourceBuiltSdk CI legs after GA

[lbussell]

After .NET 8 is released, (or whenever we are prebuilt-clean and will likely not require a re-bootstrap) we should get artifacts from the CentOSStream8_Offline_PreviousSourceBuiltSdk leg instead of the CentOSStream8_Offline_MsftSdk leg. This would let us truly simulate a distro-maintainer workflow in our CI post-release, ensuring we don't introduce any Microsoft-built binaries at any point past the initial bootstrap during .NET 8.0's servicing lifetime.

See the following line:

source-build/eng/templates/stages/release.yml

Line 82 in 12c50f7

value: CentOSStream8_Offline_MsftSdk_x64_Artifacts

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.