Add CI check to ensure the new environments are deployed before merging

!1283 (merged) was merged even if the deployment to CVMFS was not successful, causing massive troubles in the nightlies.

We should use something like https://gitlab.cern.ch/lhcb-core/ci-utils/-/blob/0eb70b41920674a153aad6ee401ac285cc865b32/data-pkg-deployment/deploy.py#L47-71 to check that the deployment went through.