ci: add TESTING_CLUSTER to PROJECT_SPACK_ROOT

[rbberger]

PR Summary

Spack upstreams are now split up by cluster.

PR Checklist

• [ ] Code is formatted. (You can use the format_spiner make target.)
• [ ] Adds a test for any bugs fixed. Adds tests for new features.
• [ ] If preparing for a new release, update the version in cmake.

[rbberger]

@Yurlungur please merge