[WIP] Add functionality for updating an existing compatibility layer

[bedroge]

Attempt to make it possible to rerun the playbook on top of an existing compat layer. Will require some changes in the installation script, as it now bind mounts an empty host dir as /cvmfs/software.eessi.io, instead it should probably just fusemount the CVMFS repo inside the container.

[bedroge]

Tried it with a bind mount from the host, but that doesn't work, as the files are then owned by the cvmfs user. This leads to all sorts of permission errors in the Prefix environment, e.g.

OSError: [Errno 95] Operation not supported: '/cvmfs/software.eessi.io/versions/2025.06/compat/linux/x86_64/var/lib/portage'

[bedroge]

bot: build repo:eessi.io-2025.06-compat instance:eessi-bot-mc-aws for:arch=x86_64/generic

[eessi-bot-aws]

New job on instance eessi-bot-mc-aws for repository eessi.io-2025.06-compat
Building on: generic
Building for: x86_64/generic
Job dir: /project/def-users/SHARED/jobs/2025.12/pr_229/113486

date	job status	comment
Dec 16 13:23:07 UTC 2025	submitted	job id 113486 awaits release by job manager
Dec 16 13:23:36 UTC 2025	released	job awaits launch by Slurm scheduler
Dec 16 13:29:19 UTC 2025	running	job 113486 is running
Dec 16 16:16:54 UTC 2025	finished	😢 FAILURE (click triangle for details) Details ✅ job output file slurm-113486.out ❌ some task failed ✅ found tarball Artefacts eessi-2025.06-compat-linux-x86_64-1765901624.tar.gz size: 1617 MiB (1696161570 bytes) entries: 180916
Dec 16 16:16:54 UTC 2025	test result	😢 FAILURE (click triangle for details) Reason EESSI test suite produced failures. ReFrame Summary [ FAILED ] Ran 23/23 test case(s) from 23 check(s) (6 failure(s), 0 expected failure(s), 0 skipped, 0 aborted) Details ✅ job output file slurm-113486.out ❌ found message matching ERROR: ❌ found message matching [\s*FAILED\s*].*Ran .* test case