* 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.