electron: add libnotify to the RPATH

Electron loads libnotify dynamically via dlopen(3), which fails if it
is not in the RPATH. This, in turn, breaks desktop notifications in,
e.g., mattermost-desktop.

pkgs/development/tools/electron/common.nix

···
     runHook postInstall
   '';
 
+  postFixup =
+    let
+      libPath = lib.makeLibraryPath [
+        libnotify
+      ];
+    in
+  base.postFixup + ''
+    patchelf \
+      --add-rpath "${libPath}" \
+      $out/libexec/electron/electron
+  '';
+
   requiredSystemFeatures = [ "big-parallel" ];
 
   passthru = {