Use git@version syntax for spack pipelines and buildcache

[cameronrutherford]

spack/spack#43859

spack/spack#38484 (comment)

Previously the git version feature of spack was a little buggy, and so this feature wasn't possible. Now that spack is able to handle this, we should attempt to properly version our packages in the GHCR based on the git branch.

This can also be extended to ReSolve and HiOp..

[cameronrutherford]

@abhyshr I realised that the current pipelines aren't actually building new versions of ExaGO at the moment, and so they'll never fail... someone else should also take over maintainer status of ExaGO's package.py ◡̈