outline: Add localRootDir to ReadWritePaths (#298892)

The service can't write to the localRootDir if its set outside of its
state directory, thus we have to manually mark it as writable.

nixos/modules/services/web-apps/outline.nix

@@ -783,6 +783,8 @@ in
         # This working directory is required to find stuff like the set of
         # onboarding files:
         WorkingDirectory = "${cfg.package}/share/outline";
+        # In case this directory is not in /var/lib/outline, it needs to be made writable explicitly
+        ReadWritePaths = [ cfg.storage.localRootDir ];
       };
     };
   };