Add cvmfs_shrinkwrap to EESSI container to enable exporting EESSI environment

[ocaisa]

With this included, a user could:

• Start the EESSI container on a new target system
• Load the environment they want to export (source EESSI, load their required modules)
• Run a script (yet to be created) that would
  • Generate a spec file for cvmfs_shrinkwrap based on the current environment
  • Run cvmfs_shrinkwrap to export the environment
  • Run mksquashfs to create a squashfs from the export
  • Tidy up

The resulting squashfs could be loaded in any container (with Linux and a supported shell). One could give a hint about how to enable host_injections with the export.

[boegel]

There's a recommendation from CernVM-FS team to use https://cvmrepo.s3.cern.ch/cvmrepo/yum/cvmfs-release-latest.noarch.rpm instead rather than https://ecsft.cern.ch/dist/cvmfs/cvmfs-release/cvmfs-release-latest.noarch.rpm, see also EESSI/eessi-demo#43 .

Doesn't have to be done in this PR, but we should take that into account...

[ocaisa]

Done in e1f6498 (along with adding squashfs-tools)

[ocaisa]

Actually the update needed for this is not trivial and there is no way (that I can see) to explore https://cvmrepo.s3.cern.ch so I'm going to have to revert that change.

[ocaisa]

To search you can go to any directory and add list.html and you can see the files/directories are contained inside. I couldn't find the src RPM that we need for RISCV support so I'm not going to include the change here (also we could version bump CVMFS itself to 2.12.x in that PR)

[ocaisa]

@bedroge I think this should be good to go?

[bedroge]

Lgtm