Prevent Unlocked / Outdated Mix Dependencies in CI

[maennchen]

Should prevent https://github.com/elixir-gettext/gettext/pull/382#issuecomment-1838814568

[coveralls]

Pull Request Test Coverage Report for Build aa465b6e275e39f8ee1e88bbe8586b36a7cea449-PR-383

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.651%

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Totals
Change from base Build e7a2c2e02b8f43d21a5c7b73f15146fde97aab91:	0.0%
Covered Lines:	531
Relevant Lines:	567

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💛 - Coveralls

[whatyouhide]

This is awesome, TIL. I didn't know about --check-locked 🙃 I'll go do that to all my OSS libraries ahahah.