Fix bug in prefetch binary package option
Fix bug in prefetch binary package option

It seems that if /usr/local/etc/pkg/repos/00_synth.conf exists, the
prefetch built packages would sometimes fail checksum or fail to fetch
good packages.  It appears that pkg(8) was checking the Synth repository
too and getting confused.  Limitiing the fetch command to the external
repository should correct the issue.

jrmarino/synth,jrmarino/synth,jrmarino/synth,jrmarino/synth