vimUtils.buildVimPlugin: don't copy the build directory with dontUnpack

With dontUnpack there is no source directory to install from, so
installPhase copied $NIX_BUILD_TOP instead: env-vars, .attrs.json, and
.attrs.sh ended up in the plugin and pulled the whole stdenv into its
closure. Skip the copy in that case and leave installing files to the
caller's phases. Plugins that unpack keep an identical installPhase.

Fixes #508465

pkgs/applications/editors/vim/plugins/utils/build-vim-plugin.nix

@@ -43,7 +43,7 @@
 
             target=$out/${rtpPath}/${path}
             mkdir -p $out/${rtpPath}
-            cp -r . $target
+            ${lib.optionalString (!(attrs.dontUnpack or false)) "cp -r . $target"}
 
             runHook postInstall
           '';

pkgs/test/vim/default.nix

@@ -25,6 +25,26 @@ lib.recurseIntoAttrs {
     vimrcConfig.packages.myVimPackage.start = with vimPlugins; [ vim-nix ];
   };
 
+  test_vim_plugin_dont_unpack =
+    let
+      plugin = vimUtils.buildVimPlugin {
+        pname = "vim-plugin-dont-unpack-test";
+        version = "0";
+        src = pkgs.writeText "probe.vim" "";
+        dontUnpack = true;
+        buildPhase = ''
+          install -D "$src" "$out/colors/probe.vim"
+        '';
+      };
+    in
+    pkgs.runCommand "vim-plugin-dont-unpack-test" { } ''
+      test -f ${plugin}/colors/probe.vim
+      for file in env-vars .attrs.json .attrs.sh; do
+        test ! -e ${plugin}/"$file"
+      done
+      mkdir -p "$out"
+    '';
+
   # test that all vimPlugins have `passthru.vimPlugin = true`
   test-all-plugins-have-vimPlugin-true =
     let