main/postmarketos-initramfs: invalidate cached rootfs after unlock

This fixes FDE unlocking after a bug surfaced from !5048 (merged). Thanks to @magdesign and @funderscore for reporting on the Matrix channels.

For downgrading the broken package on slot b, userdata installations:

# umount /boot
# mount /dev/mapper/root /sysroot
# mount /dev/mapper/userdata1 /sysroot/boot
# chroot /sysroot
# openrc sysinit
# apk add --allow-untrusted /var/cache/apk/postmarketos-initramfs-3.1.0-r2.547ef266.apk
# exit
# dd if=/sysroot/boot/boot.img of=/dev/disk/by-partlabel/boot_b
# reboot -f