cinnamon.nemo: Fix open as root

Open as root support is added in src/nemo-view.c as `pkexec nemo`.
This does not match the full path in `org.nemo.root.policy` so it
doesn't work.

Replacing with `/run/current-system/sw/bin/nemo` seems to be the
easiest solution here, otherwise to properly support nemo-with-extensions,
we likely need to patch everything that calls `pkexec nemo` to accept
external environment variables for nemo path, and pass the path via
wrapper, and we still need to substitute `org.nemo.root.policy` in
nemo-with-extensions.

pkgs/desktops/cinnamon/nemo/default.nix

@@ -67,6 +67,13 @@ stdenv.mkDerivation rec {
     "--localedir=${cinnamon-translations}/share/locale"
   ];
 
+  postInstall = ''
+    # This fixes open as root and handles nemo-with-extensions well.
+    # https://github.com/NixOS/nixpkgs/issues/297570
+    substituteInPlace $out/share/polkit-1/actions/org.nemo.root.policy \
+      --replace-fail "$out/bin/nemo" "/run/current-system/sw/bin/nemo"
+  '';
+
   preFixup = ''
     # Used for some non-fd.o icons (e.g. xapp-text-case-symbolic)
     gappsWrapperArgs+=(