Fail if `flake.nix` inputs point to a missing branch

[srid]

nix build won't necessarily catch this due to the possibility of these inputs being cached.

There's no standard way to do this. But here's one trick that works:

• Remove "ref" field (pointing to custom branch) from flake.lock for all our inputs
• Then, run nix --refresh flake archive which should recreate flake.lock, thus reverting our changes if successful, after refetching those branches.

Are there better approaches?

[srid]

This solution however is not general enough as it applies only to VCS inputs and not to say direct URL inputs (eg: tarball).