(PUP-2705) Use source permissions for fact pluginsync

The fix for PUP-1986 caused the agent to no longer apply source
permissions of pluginsync'ed content to the downloaded files. This
caused a regression with external executable facts, since they must be
executable on the agent in order for facter to execute them.

This commit re-enables using source permissions during fact pluginsync,
while continuing to ignore them for module pluginsync.