builders: limit system generations we keep around

These are quite large on aarch64-linux due to lack of compression.

Author: mweinelt

builders/boot/efi-grub.nix

@@ -3,6 +3,7 @@
     efi.canTouchEfiVariables = false;
     grub = {
       enable = true;
+      configurationLimit = 5;
       efiSupport = true;
       efiInstallAsRemovable = true;
       mirroredBoots = [