makeDesktopItem: use runCommandLocal

This derivation only creates a simple text file, so it makes sense to do
it locally.

On my setup this reduces build time from 2.2s to 1.2s.

pkgs/build-support/make-desktopitem/default.nix

@@ -1,4 +1,4 @@
-{stdenv, lib}:
+{ lib, runCommandLocal }:
 { name
 , type ? "Application"
 , exec
@@ -13,11 +13,7 @@
 , extraEntries ? null
 }:
 
-stdenv.mkDerivation {
+let
-  name = "${name}.desktop";
-
-  buildCommand = let
-
   optionalEntriesList = [{k="Icon";          v=icon;}
                          {k="Comment";       v=comment;}
                          {k="GenericName";   v=genericName;}
@@ -29,8 +25,8 @@ stdenv.mkDerivation {
 
   mkEntry = {k, v}:  k + "=" + v;
   optionalEntriesString  = lib.concatMapStringsSep "\n" mkEntry entriesToKeep;
-
 in
+runCommandLocal "${name}.desktop" {}
   ''
     mkdir -p $out/share/applications
     cat > $out/share/applications/${name}.desktop <<EOF
@@ -44,5 +40,4 @@ stdenv.mkDerivation {
     ${if extraEntries == null then ''EOF'' else ''
     ${extraEntries}
     EOF''}
-  '';
+  ''
-}