thelounge: write out default path for thelounge

The output file is found and handled by thelounge itself [1], leaving
the user free to override THELOUNGE_HOME in the environment if they
choose, but having a sensible default to make `thelounge` generally
usable in most cases.

This solution follows discussion on #70318.

[1] https://github.com/thelounge/thelounge/blob/9ef5c6c67e463c1f401e33b21dfb5641636e5ed1/src/command-line/utils.js#L56

pkgs/development/node-packages/default-v10.nix

@@ -125,5 +125,8 @@ nodePackages // {
 
   thelounge = nodePackages.thelounge.override {
     buildInputs = [ nodePackages.node-pre-gyp ];
+    postInstall = ''
+      echo /var/lib/thelounge > $out/lib/node_modules/thelounge/.thelounge_home
+    '';
   };
 }