mix-test and mix-format broken in the `nix flake check` context

[ProducerMatt]

Elixir Mix tests and formatting work, except in flake check. As @nifoc noted here:

However, nix flake check does not work, because:

• Dependencies are not copied into the drv (because they're part of the .gitignore)

       > Shall I install Hex? (if running non-interactively, use "mix local.hex --force") [Yn] ** (Mix) Could not find an SCM for dependency :ecto from Pan.MixProject
       > Could not find Hex, which is needed to build dependency :ecto