Revert "nixos/raspberrypi-builder: fix cross using buildPackages"

The commit enforces buildPackages in the builder but neglects
the fact that the builder is intended to run on the target system.
Because of that, the builder will fail when remotely building a
configuration eg. with nixops or nix-copy-closure.

This reverts commit a6ac6d00f9.

nixos/modules/system/boot/loader/raspberrypi/raspberrypi-builder.nix

@@ -3,8 +3,8 @@
 pkgs.substituteAll {
   src = ./raspberrypi-builder.sh;
   isExecutable = true;
-  inherit (pkgs.buildPackages) bash;
-  path = with pkgs.buildPackages; [coreutils gnused gnugrep];
+  inherit (pkgs) bash;
+  path = [pkgs.coreutils pkgs.gnused pkgs.gnugrep];
   firmware = pkgs.raspberrypifw;
   inherit configTxt;
 }