[llvm] test-release.sh: Only build the clang target in stage 1 and 2 (PR #72703)

[Tobias Hieta via llvm-commits]

https://github.com/tru approved this pull request.

LGTM. But feels like a bit of Deja vu. Didn't we do this at some point already? Or something very similar at least. 

https://github.com/llvm/llvm-project/pull/72703