packages: Make create-sets.sh more robust

Use ${PKG_CMD} rather than bare 'pkg' to fix the build when pkg is
not in the tools path. Provide a default in case it's not set for
some reason (e.g., running the script by hand).

Since set -- $(...) does not trigger an exit from set -e if the
command fails, this failure was silent and resulted in sets not
being built correctly if we failed to run pkg. Use a temporary
variable, which does trigger set -e, to fail correctly.

MFC after: 2 weeks
Reviewed by: sjg
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56676