pmb.chroot.initfs: adapt to new mkinitfs

Follow-up to 09794ef8 - initfs file does not have flavor now, too.

Without this 'pmbootstrap initfs extract' fails with:

(rootfs_samsung-klte) % cp /boot/initramfs-postmarketos-qcom-msm8974 /tmp/initfs-extracted/_initfs.gz
cp: cannot stat '/boot/initramfs-postmarketos-qcom-msm8974': No such file or directory

Test plan: run pmbootstrap initfs ls or pmbootstrap initfs extract on various devices