in pleroma checkout:
- grab version: `rg 'version: ' mix.exs`

in default.nix:
- update `rev` and recompute sha256.

in pleroma checkout:
- `mix2nix > mix.nix`

in nix repo:
- cp the new mix.nix here.
- move majic from mix.nix -> default.nix and add:
        - buildInputs = [ file ];
- update `mixNixDeps` in default.nix:
        - grab the version from pleroma/mix.exs or mix.lock
- redundant?: inside mix.nix, change base64url to use buildRebar3 instead of buildMix