nixos/stage-1: Fix references to readlink/basename

Fixes references coming from the mdadm udev rules.

This addresses #12722 (mdadm udev rules have references to /usr/bin) but
still won't fix the warning, though (if we want to fix the warnings, we
will have to patch the udev rules generater in services/hardware/udev).

For common mdraid functionality, this shouldn't fix anything, because
the wrong references seem to only apply to containers, see these
(wrapped) lines from ${mdadm}/lib/udev/rules.d/63-md-raid-arrays.rules:

  # Tell systemd to run mdmon for our container, if we need it.
  ENV{MD_LEVEL}=="raid[1-9]*",
    ENV{MD_CONTAINER}=="?*",
  PROGRAM="/usr/bin/readlink $env{MD_CONTAINER}",
    ENV{MD_MON_THIS}="%c"
    ENV{MD_MON_THIS}=="?*",
    PROGRAM="/usr/bin/basename $env{MD_MON_THIS}",
    ENV{SYSTEMD_WANTS}+="mdmon@%c.service"

Signed-off-by: aszlig <aszlig@redmoonstudios.org>