Discontinue the mingw target in the nix build

[neilmayhew]

Description

This should fix the CI failures we've been having for a while (due to the cddl executable not being available in the mingw build environment.

Fixes #4074

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated. If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages. New section is never added with the code changes. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[neilmayhew]

The Hydra CI succeeded, except for one test on Darwin x86. Maybe this is a flaky test? The failed build result is actually cached from an earlier PR.

[lehins]

Hydra CI looks a lot less red then before. Very nice!

[lehins]

Maybe this is a flaky test?

That was a flaky test.