Enable systemd for GNOME ui packages

@craftyguy thinks we should enable systemd for gnome/gnome-mobile by default. plamo and phosh also do this. Users who want to use openrc can still disable it with pmbootstrap config systemd never

cc @pabloyoyoista @Newbyte

old summary

(this patch was originally sent to fix some unrelated issue below)

When building with pmabootstrap config systemd default, pmboostrap install fails with:

(rootfs_oneplus-enchilada) % setup-timezone -i Europe/Berlin
/bin/sh: setup-timezone: not found

Tested locally and this fixes the build step.