Validate tags in CI

[bgamari]

Currently it's very easy to overlook or otherwise mess up the tags when adding a new tool version. CI should ideally perform some consistency checks to catch this failure mode:

• tags should be valid (matching a regular expression capturing known tags)
• there should be one Latest and one LatestPrerelease, and one Recommended version for each tool
• each ghc tool should have a base-... tag

[hasufell]

CI already does that:

• ghcup-metadata/ghcup-gen/Validate.hs

  Line 126 in 1d366ed

  checkUniqueTags tool = do

• ghcup-metadata/.github/workflows/test.yaml

  Line 75 in 1d366ed

  ghcup-gen -- check -f ghcup-${{ env.YAML_VER }}.yaml

That can probably be extended.

[hasufell]

Done: #137

Example pipeline on prereleases: https://github.com/haskell/ghcup-metadata/actions/runs/6369219389/job/17289264412#step:10:18