[Release] Don't build during test-release.sh Phase 3 install (#82001)

As described in [test-release.sh ninja install does builds in Phase
3](https://github.com/llvm/llvm-project/issues/80999), considerable
parts of Phase 3 of a `test-release.sh` build are run by `ninja
install`, ignoring both `$Verbose` and the parallelism set via `-j NUM`.

This patches fixes this by not specifying any explicit build target for
Phase 3, thus running the full build as usual.

Tested on `sparc64-unknown-linux-gnu`.