in default.nix:
  update `rev` and recompute sha256.
    use nix to-sri sha256:<expected>

run mix2nix inside the pleroma git root and pipe the output into mix.nix
  inside default.nix, update all git mix deps
  inside mix.nix, change base64url to use buildRebar3 instead of buildMix

move majic from mix.nix -> default.nix and add:
  buildInputs = [ file ];