make-disk-image: Reintroduce the installBootLoader argument

installBootLoader was removed in
f1708a9d7d79e2bf2961fc648625578b23b3460f; probably by mistake, since
only the logic was removed, not the argument.

nixos/lib/make-disk-image.nix

@@ -44,6 +44,9 @@
   #   most likely fails as GRUB will probably refuse to install.
   partitionTableType ? "legacy"
 
+, # Whether to invoke `switch-to-configuration boot` during image creation
+  installBootLoader ? true
+
 , # The root file system type.
   fsType ? "ext4"
 
@@ -368,11 +371,13 @@ in pkgs.vmTools.runInLinuxVM (
         cp ${configFile} /mnt/etc/nixos/configuration.nix
       ''}
 
-      # Set up core system link, GRUB, etc.
-      NIXOS_INSTALL_BOOTLOADER=1 nixos-enter --root $mountPoint -- /nix/var/nix/profiles/system/bin/switch-to-configuration boot
+      ${lib.optionalString installBootLoader ''
+        # Set up core system link, GRUB, etc.
+        NIXOS_INSTALL_BOOTLOADER=1 nixos-enter --root $mountPoint -- /nix/var/nix/profiles/system/bin/switch-to-configuration boot
 
-      # The above scripts will generate a random machine-id and we don't want to bake a single ID into all our images
-      rm -f $mountPoint/etc/machine-id
+        # The above scripts will generate a random machine-id and we don't want to bake a single ID into all our images
+        rm -f $mountPoint/etc/machine-id
+      ''}
 
       # Set the ownerships of the contents. The modes are set in preVM.
       # No globbing on targets, so no need to set -f