feat(nix): add settings to automatically run the gc

this should make it slightly easier to use nix
without manually running the GC,
but also doesn't run anything in the bg for this
because that could be poor UX

defaults/nix.nix

@@ -25,6 +25,13 @@ in
     # source: https://nix.dev/manual/nix/2.26/command-ref/new-cli/nix3-store-optimise
     nix.settings.auto-optimise-store = true;
 
+    # trigger the garbage collector automatically
+    # when free space in /nix/store goes below this
+    nix.settings.min-free = 1000000000; # 1GB
+    # free up to this many bytes in an automatic invocation
+    # to avoid it taking too long
+    nix.settings.max-free = 5000000000; # 5GB
+
     environment.systemPackages =
       with pkgs;
       lib.mkIf config.nixowos.allowAddingPackages [