fix(sterni/code.sterni.lv): gc after mirroring repositories

Repositories (especially nixpkgs) WILL grow to ridiculous sizes
otherwise, killing cgit performance in the process.

Change-Id: I3bef3e5dc5a61152e89fd53e31f14e78193a4888
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12793
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/machines/ingeborg/http/code.sterni.lv.nix

@@ -260,6 +260,9 @@ in
                   cd "${path}"
 
                   git fetch "${repo.upstream}" '+refs/*:refs/*' --prune
+
+                  # no auto gc here and we're not pruning for nothing…
+                  git gc
                 '';
 
               serviceConfig = {