cache CernVM-FS package in GA workflow checking on pilot repo

[boegel]

It seems like the CernVM-FS package repo doesn't like being hammered by GitHub Actions:

Err:24 https://cvmrepo.web.cern.ch/cvmrepo/apt focal-prod InRelease
  403  Forbidden [IP: 188.185.90.87 443]
Reading package lists...
E: Failed to fetch http://cvmrepo.web.cern.ch/cvmrepo/apt/dists/focal-prod/InRelease  403  Forbidden [IP: 188.185.90.87 443]
E: The repository 'http://cvmrepo.web.cern.ch/cvmrepo/apt focal-prod InRelease' is not signed.
Reading package lists...
Building dependency tree...
Reading state information...
E: Unable to locate package cvmfs

So we need to try and avoid that the packages are downloaded again and again, there's some support for caching in GitHub Actions that may help here...