chore(tazjin/polyanka): turn on automatic-gc module

Change-Id: I0521ded37ea047124c117b07d01016a6f0a1d4ac
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7657
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

users/tazjin/nixos/polyanka/default.nix

@@ -97,6 +97,15 @@ in
     ];
   };
 
+  # Automatically collect garbage from the Nix store.
+  services.depot.automatic-gc = {
+    enable = true;
+    interval = "daily";
+    diskThreshold = 2; # GiB
+    maxFreed = 8; # GiB
+    preserveGenerations = "14d";
+  };
+
   # List packages installed in system profile. To search, run:
   # $ nix search wget
   environment.systemPackages = with pkgs; [