How to enable LUKS disk decryption on a GNU Guix System with a key file stored on a USB drive

I followed the advice from https://unix.stackexchange.com/questions/786259/guix-how-to-set-one-file-system-as-a-dependency-of-another-or-mount-one-devi , but it's halfway working for me. In my case, I still have to type the password before the GRUB menu. The second time during Linux boot, decryption happens automatically, so part of it succeeds.. When I ran guix system build, I saw a warning like 'possibly unbound variable mount-file-system'. My configuration right now

(apply raw-initrd
       (append (list file-systems) all-key-arguments
               (list
                #:pre-mount #~(begin
                                (use-modules ((gnu build
                                                   file-systems)
                                              #:select (mount-file-system)))
                                (mount-file-system (file-system
                                                     (mount-point
                                                      "/early-mnt")
                                                     (device (file-system-label
                                                              "USBDRIVE"))
                                                     (type "ext4"))
                                                   ;; This is needed otherwise the mount point
                                                   ;; will be prefixed by '/root'
                                                   #:root ""))
                #:helper-packages (append helper-packages
                                          (file-system-packages
                                           file-systems
                                           #:volatile-root?
                                           volatile-root?)
                                          (if keyboard-layout
                                              (list
                                               loadkeys-static)
                                              '())))))))