The referenced commit had surfaced a bug with the unlocking code, which
would attempt to change the output of find_root_partition while the
cached PMOS_ROOT is stale and active. Clear this variable to force
find_root_partition to find the new rootfs value.
Fixes: 7030e08e7b ("postmarketos-initramfs: check for root subpartition (MR 5048)")
[ci:skip-build]: already built successfully in CI