Extend and simplify CI setup

This does a couple of things. In the first commit:

1. This removes the recently introduced blubber setup for running the unit test suite, and instead runs it directly in a CI job. On further reflection, anyone developing mercurius is going to have a local go toolchain of the appropriate version anyway (at least if they have a base distribution of 1.21 or later [0]).
2. This adds a check stage, which currently runs go vet and go fmt. The latter is initially set to non-blocking (allow_failure).
3. The build job no longer stores binary artifacts, which we do not need given our use of debian packages.

In the second commit, the only two check-stage (fmt) issues are fixed.

You can see the effect of the not-yet-fixed format issues in [1].

[0] https://tip.golang.org/doc/toolchain

[1] https://gitlab.wikimedia.org/repos/sre/mercurius/-/pipelines/85789