refactor(wpcarro/nixos): Define common.services

DRY things up with this mixin.

Change-Id: I5791a50b01902734dff91d391e1aa90a21ce4fbb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4600
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

users/wpcarro/common.nix

@@ -1,6 +1,16 @@
 { pkgs, ... }:
 
 {
+  services = {
+    depot.automatic-gc = {
+      enable = true;
+      interval = "1 hour";
+      diskThreshold = 16; # GiB
+      maxFreed = 10; # GiB
+      preserveGenerations = "14d";
+    };
+  };
+
   # Command-line tools I commonly used and want available on most (or all) of my
   # machines.
   shell-utils = with pkgs; [