Re-work of mucoll-spack to enable build of releases fully on github

[madbaron]

In preparation for the next software release, this PR updates mucoll-spack to enable the complete build of releases within the github workflows.

This relies on an intermediate spack environment (key4hep-dev-base) by @tmadlener that contains the external dependencies of the key4hep stack. This environment is used to build an intermediate docker image (called mucoll-minimal) that is subsequently picked up to build the muon collider stack.

The CI is configured so that whenever the dockerfiles, or the spack configuration is changed, only the affected images (and those downstream) will be re-built.
The resulting docker images are automatically published and linked to the github page.

The complete workflow takes about 8 hours to complete on the github runners.

Please note that deploying the workflows to re-build the releases requires (without using external triggers) the dockerfiles to be bundled together with mucoll-spack, which is different from what we did with 2.9.
Personally I think this is ok (perhaps even a little better for discoverability) but please let me know if you disagree.

Adding a few people as reviewers!

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Some notes for the future:

• the preparation of the mucoll-minimal image (which is really a key4hep-minimal) would be best moved to key4hep-spack as it is completely generic
• we should set up a cron-triggered workflow to run as nightly build with the latest versions of all packages
• the previous concretize workflows have been removed, as they are part of the docker image building process. They could be split off to a separate step of the workflow if useful
• we can review the list of packages to be compiled (i.e. all event generators) to make the resulting image a little slimmer

[tmadlener]

I didn't have time to look in-depth, I just have one comment already about the new grouping of packages into mucoll-stack variants.

[madbaron]

A couple of notes:

• The image building pipeline is not expected to complete successfully on a PR (the github token is not given write access in PRs to public repositories, per github policy). If people would like to take a look at the outputs, the corresponding workflow was executed in my fork: https://github.com/madbaron/mucoll-spack/actions/runs/12018805366
• Since the build workflow won't succeed on PRs, I have re-introduced the concretisation checks (and plan to make them a bit more granular once a version of the various docker image is present in the muoncollidersoft registry