Set up CI on CSCS machines

[GeigerJ2]

We use that to iterate on the configuration YAML file and test the setup.

[GeigerJ2]

From the investigation of @DropD why apt was failing:

It looks like apt update gives the exact same output between working and failing jobs. My best guess is that the apt repo was not updated in either case. Now it seems that the url for libabsl20220623 in the old repo has actually gone out of date and is returning a 404 - Not Found now, where it didn't before.

All of this seems to trace back to

Err:1 http://deb.debian.org/debian bookworm InRelease
Couldn't create temporary file /tmp/apt.conf.BfDbRn for passing config to apt-key

Which then leads to

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: http://deb.debian.org/debian bookworm InRelease: Couldn't create temporary file /tmp/apt.conf.BfDbRn for passing config to apt-key

Which means that "we don't have permission to write to /tmp, which makes the signature verification process impossible and therefore apt refuses to update".

Thus, we added:

variables:
    TMPDIR: '/var/tmp'

To the CI YAML file for now, until CSCS figures out why there's currently no write access to /tmp during CI.