pmb.install: Check for alpine-conf version

When running pmbootstrap install it would get stuck at

(396601) [13:31:56] NOTE: No valid keymap specified for device
(396601) [13:31:56] (rootfs_zte-axon7) % setup-timezone -z Europe/Berlin
Which timezone are you in? ('?' for list) [UTC] (397306)

This is because the -z argument no longer exists in setup-timezone:

usage: setup-timezone [-h] [-k|-i] [TIMEZONE]

Sets the timezone for the system.

options:
 -h  Show this help
 -i  Install tzdata and symlink instead of making a copy
 -k  Keep previous copies of tzdata

TIMEZONE is relative /usr/share/zoneinfo.

If TIMEZONE is not specified user will be prompted.

This commit makes it check the alpine-conf version so we can run pmbootstarp install both on the edge and stable branch.

fixes #2128 (closed)