Provide reproducibility between builds by building pkg(8) from ports,
instead of using pkg-bootstrap.

This should resolve a problem that was discovered during 10.0-BETA4
freebsd-update(8) builds,

Reported by:	secteam (delphij)
MFC after:	3 days
X-MFC-to-10:	possibly immediately, pending freebsd-update(8) builds
Sponsored by:	The FreeBSD Foundation