fix(whitby): Disable git's gc.autoDetach feature

This feature can cause object removal to happen while the git folder
is in use in Buildkite, causing CI to fail semi-reegularly.

Change-Id: Ide1a9b2f1761be029e97a058c1983b4cff5e27bf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2285
Tested-by: BuildkiteCI
Reviewed-by: multi <depot@in-addr.xyz>

ops/nixos/whitby/default.nix

@@ -147,6 +147,12 @@ in lib.fix(self: {
     '';
   };
 
+  # Disable background git gc system-wide, as it has a tendency to break CI.
+  environment.etc."gitconfig".source = depot.third_party.writeText "gitconfig" ''
+    [gc]
+    autoDetach = false
+  '';
+
   time.timeZone = "UTC";
 
   nix = {