postmarketos-mkinitfs: generates bogus initfs when "cpio" package is installed

To reproduce (e.g. on pinephone):

$ mkinitfs -o /boot/initramfs-postmarketos-allwinner 5.7.0
$ ls -l /boot/initramfs-postmarketos-allwinner*
-rw-r--r-- 1 root root 1454960 Sep 29 20:24 /boot/initramfs-postmarketos-allwinner
-rw-r--r-- 1 root root 6823124 Sep 29 20:24 /boot/initramfs-postmarketos-allwinner-extra
# file size is sane

$ apk add cpio
$ mkinitfs -o /boot/initramfs-postmarketos-allwinner 5.7.0
$ ls -l /boot/initramfs-postmarketos-allwinner*
-rw-r--r-- 1 root root 91 Sep 29 20:25 /boot/initramfs-postmarketos-allwinner
-rw-r--r-- 1 root root 91 Sep 29 20:25 /boot/initramfs-postmarketos-allwinner-extra
# files are basically empty cpio archives

Basically for whatever reason our cpio usage only works with busybox cpio but not with gnu cpio.