Add test for missing make vendor run

[bwplotka]

Proposal

We constantly forget to do go mod vendor, no CI check for this?

[bernot-dev]

Usually the build fails if you don't run go mod vendor. We could add tidy and vendor to the regen target, I suppose?

[pintohutch]

It's actually a part of the make regen target already, which calls the presubmit.sh script to execute various commands.

[pintohutch]

Oh - and in Github CI in the validate step, where it ensures there's no diff after running.

Could potentially be improved though :)

[bwplotka]

None of those works on prometheus, I think you refer to prometheus-engine.

Anyone, vendoring has been moved to our mirror so no need for this anymore!