make-disk-image: add systemdMinimal to PATH

this is needed to setup /tmp inside the chroot. which is needed by
systemd-boot to sucessfully install

lassulus ed18db01 9d6d47a8

Changed files
+1
nixos
+1
nixos/lib/make-disk-image.nix
···
config.system.build.nixos-install
config.system.build.nixos-enter
nix
+
systemdMinimal
] ++ stdenv.initialPath);
# I'm preserving the line below because I'm going to search for it across nixpkgs to consolidate