Define package using mixRelease. (#40)

The previous definition resulted in `mix deps.get` making calls to the
internet (to downloaded packages from Hex) which fails when building on
Linux (which restricts network access in the build sandbox).

By using mixRelease, we can use the mixFetchDeps helper which
effectively converts Hex packages into separate Nix derivations.

flake.nix

@@ -18,7 +18,7 @@
       pkgs = import nixpkgs {inherit system;};
       beamPackages = pkgs.beam.packages.erlang_25;
       elixir = beamPackages.elixir_1_14;
-      hex = beamPackages.hex;
+      src = pkgs.nix-gitignore.gitignoreSource [] ./.;
     in {
       # `nix develop`.
       devShells = {
@@ -32,23 +32,25 @@
       formatter = pkgs.alejandra;
       # `nix build`.
       packages = {
-        build-pipeline = pkgs.stdenvNoCC.mkDerivation {
-          inherit pname version;
-          src = pkgs.nix-gitignore.gitignoreSource [] ./.;
-          nativeBuildInputs = [elixir pkgs.makeWrapper];
-          buildPhase = ''
-            # Expose Nix's hex to Mix.
-            export MIX_PATH="${hex}/lib/erlang/lib/hex/ebin"
-            export HOME=$PWD/.hex
-            mkdir -p $HOME
-            mix deps.get
+        build-pipeline = beamPackages.mixRelease {
+          inherit pname src version;
+          mixFodDeps = beamPackages.fetchMixDeps {
+            inherit pname src version;
+            hash = "sha256-H7yiBHoxuiqWcNbWwPU5X0Nnv8f6nM8z/ZAfZAGPZjE=";
+            mixEnv = "prod";
+          };
+          postBuild = ''
             env MIX_ENV=prod mix escript.build
           '';
-          installPhase = ''
+          postInstall = ''
+            # Install the `bp` escript.
             mkdir -p $out/bin
             cp bp $out/bin/bp
             wrapProgram $out/bin/bp \
               --prefix PATH : ${pkgs.lib.makeBinPath [beamPackages.erlang]}
+
+            # Clean up the Mix release stuff, most of which we don't need.
+            rm -r $out/{lib,releases,erts-*,bin/build_pipeline}
           '';
         };
         default = self.packages.${system}.build-pipeline;