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

release/scripts/pkg-stage.sh

@@ -25,7 +25,7 @@ REVISION="${2}"
 . "${1}" || exit 1
 
 if [ ! -x /usr/local/sbin/pkg ]; then
-	/usr/sbin/pkg bootstrap	
+	/usr/bin/make -C /usr/ports/ports-mgmt/pkg install clean
 fi
 
 /bin/mkdir -p ${PKG_CACHEDIR}