pmbootstrap: run partprobe after repartitioning (MR 2473)

On some kernels (notably, WSL2) parted fails to inform the kernel
about the new partition layout, leading to errors when trying to
create the new filesystems.

This change runs an additional `partprobe` after updating the
partition table, informing the kernel about the new layout.

Fixes: #2422
Signed-off-by: Dominik Bayerl <dominik.bayerl@carissma.eu>
[caleb: move call so it runs after partition_cgpt() as well]
Signed-off-by: Caleb Connolly <caleb@postmarketos.org>