Feature #5855 -- fix withenv call in freebsd package provider

Qualify 'withenv' in the provider, since it is part of an entirely different
part of puppet, not a method on this provider. This closes #5855.

Signed-off-by: fredrik-eriksson <fredrik.eriksson@loopia.se>
Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>