To save space we compress our packages for the ISO with xz, but the
default for pkgmk should still be gz, because of performance issues
with xz. To solve this we overwrite /etc/pkgmk.conf in our build
environment with a spezial one that's part of our iso.git. As a side
effect we can set now other things like MAKEFLAGS and PKGMK_WORK_DIR
as well.
