linux-initrd: Ensure that the guile used in the initrd is referenced.

linux-initrd: Ensure that the guile used in the initrd is referenced.

By referencing guile from the initrd output explicitly, it will be present in
the store when this initrd is used. If the exact guile used within the initrd
isn't present in the store, then after root is switched during the boot
process, loading modules (such as (ice-9 popen)) won't work.

This fixes guix-patches bug #28399, "Fix mysql activation, and add a basic
test".

* gnu/system/linux-initrd.scm (expression->initrd)[builder]: Write out a file
  called references in to the initrd output, which includes the store path for
  guile.