mailpit: fix build by splitting off nodejs part

pkgs/servers/mail/mailpit/default.nix

@@ -1,4 +1,5 @@
 { lib
+, stdenv
 , buildGoModule
 , nodejs
 , python3
@@ -8,8 +9,8 @@
 , fetchNpmDeps
 }:
 
-buildGoModule rec {
-  pname = "mailpit";
+let
+
   version = "1.7.1";
 
   src = fetchFromGitHub {
@@ -19,23 +20,45 @@ buildGoModule rec {
     hash = "sha256-jT9QE0ikp9cJlT8qtfPPjKOUuqWyQk94D3UbkyaGXa8=";
   };
 
-  vendorHash = "sha256-XBYIO7fdo5EahJB7EcAuY9SGKZb8dsvoJHp/D5LO5Qo=";
+  # Separate derivation, because if we mix this in buildGoModule, the separate
+  # go-modules build inherits specific attributes and fails. Getting that to
+  # work is hackier than just splitting the build.
+  ui = stdenv.mkDerivation {
+    pname = "mailpit-ui";
+    inherit src version;
 
-  npmDeps = fetchNpmDeps {
-    inherit src;
-    hash = "sha256-6VCs8125fTJkZW+eZgK56j7ccK8tcGhIXiq2HkYp4XM=";
+    npmDeps = fetchNpmDeps {
+      inherit src;
+      hash = "sha256-6VCs8125fTJkZW+eZgK56j7ccK8tcGhIXiq2HkYp4XM=";
+    };
+
+    nativeBuildInputs = [ nodejs python3 libtool npmHooks.npmConfigHook ];
+
+    buildPhase = ''
+      npm run package
+    '';
+
+    installPhase = ''
+      mv server/ui/dist $out
+    '';
   };
 
-  nativeBuildInputs = [ nodejs python3 libtool npmHooks.npmConfigHook ];
+in
 
-  preBuild = ''
-    npm run package
-  '';
+buildGoModule {
+  pname = "mailpit";
+  inherit src version;
+
+  vendorHash = "sha256-XBYIO7fdo5EahJB7EcAuY9SGKZb8dsvoJHp/D5LO5Qo=";
 
   CGO_ENABLED = 0;
 
   ldflags = [ "-s" "-w" "-X github.com/axllent/mailpit/config.Version=${version}" ];
 
+  preBuild = ''
+    cp -r ${ui} server/ui/dist
+  '';
+
   meta = with lib; {
     description = "An email and SMTP testing tool with API for developers";
     homepage = "https://github.com/axllent/mailpit";