Spack GitHub action

[cameronrutherford]

This modifies exago's spack package, so spack/spack#40188 should be merged before this, and submodule updated accordingly.

Closes #11

[cameronrutherford]

While we still have other work to do with submodules, these builds should catch some errors we are seeing in xSDK pipelines and so are useful in their current state. This will be improved in the future.

[cameronrutherford]

Currently each of the spack builds take at least 30min of GitHub runner time, so we should both minimize the set of configurations we test, and also try to only run CI infrequently (maybe only on merges into develop from now on?)

[cameronrutherford]

https://github.com/pnnl/ExaGO/actions/runs/6279332431/job/17054811101?pr=15 - really interesting bug here. This is probably one of the first times that someone has built exago in this minimal config, so we have an error that we don't import os in our package.

Normally something like hiop or another package would conveniently import this for us, but we finally hit a build configuration where this wasn't the case :)

See spack PR for the fix that is used in this PR (ideally Spack PR is merged before this) spack/spack#40188

[cameronrutherford]

xsdk-project/xsdk-issues#220 - finally reproduced this issue in the pipelines here. Just a matter of actually fixing the issue now.

[cameronrutherford]

@ryandanehy and @abhyshr I think this is ready to go when CI passes, so I would love at least one more review of this finished product before merging.

This also resolves issues with xSDK build failures, so good to get this merged soon.

[cameronrutherford]

See #17 for issue identified when building exago~tests

[cameronrutherford]

See #18 and LLNL/hiop#662 for issues with exago~mpi+hiop

[abhyshr]

Looks good to me.