mpd-mpris: use `install` to install mpd-mpris.service

pkgs/tools/audio/mpd-mpris/default.nix

···
   subPackages = [ "cmd/${pname}" ];
 
   postInstall = ''
-    mkdir -p $out/lib/systemd/user
-    cp mpd-mpris.service $out/lib/systemd/user
+    install -Dm644 mpd-mpris.service $out/lib/systemd/user/mpd-mpris.service
   '';
 
   meta = with lib; {