mattermost: make it work with nix-update

this mirrors a change in the nixpkgs definition: the nix-update script
has a hardcoded list of attributes it will update. We can re-use one of
them to make it update mattermost's web frontend at the same time as it
updates mattermost itself.

The list of attribute names is here:
  https://github.com/Mic92/nix-update/tree/1.3.1?tab=readme-ov-file#features
original nixpkgs commit by numinit was
  1451a58a57e1bd1592460268bdde30cf72923010
  1451a58a57

pkgs/mattermost.nix

@@ -45,6 +45,7 @@ buildGoModule rec {
   '';
 
   subPackages = [ "cmd/mattermost" ];
+  offlineCache = webapp;
 
   tags = [ "production" ];