linux-initrd: Use (guix cpio) instead of GNU cpio.

* gnu/build/linux-initrd.scm (write-cpio-archive): Remove 'open-pipe*' and
  related calls.  Compute list of files in 'files' variable.  Use
  'cpio:write-cpio-archive'.  Remove #:cpio parameter.
  (build-initrd): Remove #:cpio parameter.
* gnu/system/linux-initrd.scm (expression->initrd): Likewise, and adjust
  BUILDER accordingly.  Add (guix cpio) to #:modules.