nextcloud: create admin user via occ

nixpkgs removed services.nextcloud.config.{adminuser,adminpassFile} and
now installs with --disable-admin-user. Add a nextcloud-admin-user oneshot
that creates the admin account from the vars secret after nextcloud-setup.

modules/clan/nextcloud/tests/vm/default.nix

@@ -31,7 +31,7 @@
 
     server.wait_for_unit("phpfpm-nextcloud.service")
 
-    # Check that garage is running
-    server.succeed("systemctl status phpfpm-nextcloud.service")
+    # Admin account exists and is in the admin group
+    server.succeed("nextcloud-occ user:info admin --output=json | grep -q '\"groups\":\\[\"admin\"\\]'")
   '';
 }