Set up a GHCR Spack build cache to speed up spack pipelines

[cameronrutherford]

Currently spack pipelines take about 15 minutes each. If we were to set up a binary mirror using the GHCR, then we could speed up builds substantially.

In addition to this, it would be good to run spack test in those pipelines. This might require slight modifications to the HiOp package, but shouldn't be too hard. See spack/spack#35751 for some changes that started in this direction.

[cameronrutherford]

This would resolve the issue seen in #692... I will work on this soon but just documenting the issues

cc @nychiang

[nychiang]

@cameronrutherford Thanks! You are the best! 👍

[nychiang]

@cameronrutherford
You fixed this issue in PR #695 , didn't you? BTW, would you mind pointing us where we can find the documentation for using GHCR for ExaGO, e.g., steps to create the docker image and linked it with CI pipeline. Cheers!