github actions: ensure our apt cache is up2date

github actions: ensure our apt cache is up2date

Otherwise we might end up with outdated mirror information:

| Err:77 http://azure.archive.ubuntu.com/ubuntu focal-updates/main amd64 lintian all 2.62.0ubuntu2
|   404  Not Found [IP: 40.119.46.219 80]
| Fetched 4172 kB in 4s (951 kB/s)
| E: Failed to fetch http://azure.archive.ubuntu.com/ubuntu/pool/main/l/lintian/lintian_2.62.0ubuntu2_all.deb  404  Not Found [IP: 40.119.46.219 80]
| E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

Gbp-Dch: Ignore