refactor(ops/cgit): make user configurable

on whitby, cgit runs as the gerrit user to get access to serving
gerrit's repositories directly.

on other machines (e.g. sanduny) this isn't necessary, as we have a
world-readable depot replica.

Change-Id: Ibf7e7cc08e5909e0fa182e561ab0cb472188edcb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5932
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

ops/machines/whitby/default.nix

@@ -413,7 +413,11 @@ in
     nixery.enable = true;
 
     # Run cgit & josh to serve git
-    cgit.enable = true;
+    cgit = {
+      enable = true;
+      user = "git"; # run as the same user as gerrit
+    };
+
     josh.enable = true;
 
     # Configure backups to GleSYS