Make literal spack commit available in spack view

[msimberg]

Similar to #199, this makes the literal spack commit specified in the recipe available as UENV_SPACK_COMMIT_LITERAL in a spack view. However, UENV_SPACK_COMMIT stays as the commit hash as before.

I've now crammed everything into the --spack command line argument of envvars.py. Searching around a bit, I think in theory commas are allowed in e.g. branch names, and could therefore end up breaking the parsing (which relies on the repo, literal commit, and commit hash being separated by commas), but it seems like in practice one should not use commas, and I've certainly never seen it in the wild. Just pointing it out in case someone thinks it's worth handling that.

I've gone with the name "commit literal" as suggested by @bcumming. I think it fits as it's the "literal" that was used to configure the recipe. I'm not sure if there's a common git term for anything that can be checked out that could be a better option? I considered "reference" since a branch or tag is a reference, but if I understand correctly a commit hash is not a reference (but a commit hash is allowed to be used here), so it doesn't quite fit.

Edit: this is now using "ref" and UENV_SPACK_REF for what is called "literal" above.

With a freshly built stack:

$ uenv start --view spack store.squashfs
loading the view cuda-12.6:spack
$ printenv | grep '^UENV_SPACK'
UENV_SPACK_COMMIT=ed1dbea77b442cd55848b11e633f6626e83a999d
UENV_SPACK_REF=develop
UENV_SPACK_CONFIG_PATH=/user-environment/config
UENV_SPACK_URL=https://github.com/spack/spack.git