default nix.gc.automatic to true

Signed-off-by: Kiara Grouwstra <kiara@procolix.eu>

infra/common/nixos/default.nix

@@ -16,6 +16,7 @@ in
   i18n.defaultLocale = "en_US.UTF-8";
   system.stateVersion = "24.05"; # do not change
   nixpkgs.hostPlatform = mkDefault "x86_64-linux";
+  nix.gc.automatic = mkDefault true;
 
   ## This is just nice to have, but it is also particularly important for the
   ## Forgejo CI runners because the Nix configuration in the actions is directly

machines/dev/forgejo-ci/forgejo-actions-runner.nix

@@ -44,4 +44,7 @@
 
   ## For the Docker mode of the runner.
   virtualisation.docker.enable = true;
+
+  # we should probably prevent CI from trashing CI builds
+  nix.gc.automatic = false;
 }