Lock on feature branch becomes invalid after merge

[roberth]

Describe the bug

This happened because...

Steps To Reproduce

1. flake-parts had a feature branch
2. arion used the feature branch
3. the feature branch was merged
4. nix is unable to download the feature branch (as reported in Problem with easyOverlay: HTTP error 422 hercules-ci/arion#178)

Expected behavior

• Used the pinned commit sha
• When running any of the nix flake lock update commands, warn that the branch is gone

nix-env --version output

Additional context

I'm aware that shas of rebased branches may or may not be downloadable, but the error message coming back from github clearly indicates that the branch could not be found.

Priorities

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