fix crash when running running `zap -m` after `zap -p`
zap -m
crashes when the /packages
directory does not exist.
$ pmb zap -m
[23:26:58] Shutdown complete
[23:26:58] Remove /var/pmbootstrap/chroot_native? (y/n) [n]:
[23:26:59] Done
$ pmb zap -p
[23:27:00] Shutdown complete
[23:27:00] Remove /var/pmbootstrap/chroot_native? (y/n) [n]: y
[23:27:02] Remove /var/pmbootstrap/packages? (y/n) [n]: y
[23:27:03] Done
$ pmb zap -m
[23:27:04] Shutdown complete
[23:27:04] ERROR: [Errno 2] No such file or directory: '/var/pmbootstrap/packages'
[23:27:04] Run 'pmbootstrap log' for details.
[23:27:04] See also: <https://postmarketos.org/troubleshooting>