ci: pin version for pkg check

[Borda]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did all existing and newly added tests pass locally?

What does this PR do?

freeze for 6.0 before we resolve:

Checking dist/lightning_utilities-0.12.0.dev0-py3-none-any.whl: ERROR    InvalidDistribution: Invalid distribution metadata: unrecognized or    
         malformed field 'license-file'   

PR review

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

---

📚 Documentation preview 📚: https://lit-utilities--356.org.readthedocs.build/en/356/