kbd-util: allow to override the default keymap directories

kbd-util: allow to override the default keymap directories

This introduces $SYSTEMD_KEYMAP_DIRECTORIES environment variable to
override the hardcoded keymap directories.

I think it is not necessary to provide the first class configuration
option for controlling the keymap directories, but it is not good to
hardcode the paths. So, let's introduce an environment variable to
override that.

Prompted by #31759.

Closes #31759.