Draft: Salsa CI: Extend builds and upgrades

See commits for details.

Also compare to:

• https://salsa.debian.org/mariadb-team/mariadb-server/-/merge_requests/116
• https://salsa.debian.org/mariadb-team/galera-4/-/merge_requests/34

Ubuntu tests are added on top of this in https://salsa.debian.org/debian/entr/-/merge_requests/13.