nixos/cgit: use alias option for assets

This makes it easier to replace an asset by just setting the Nginx
virtual host's `locations."= robots.txt".alias`. Previously you had to
either replace the extraConfig with mkForce or clear it and use the
`alias` option.

nixos/modules/services/networking/cgit.nix

@@ -256,9 +256,7 @@ in
             (genAttrs' [ "cgit.css" "cgit.png" "favicon.ico" "robots.txt" ] (
               fileName:
               lib.nameValuePair "= ${stripLocation cfg}/${fileName}" {
-                extraConfig = ''
-                  alias ${cfg.package}/cgit/${fileName};
-                '';
+                alias = lib.mkDefault "${cfg.package}/cgit/${fileName}";
               }
             ))
             // {