bsdinstall: use the correct DISTDIR for fetching local distfiles

fetchmissingdists naturally sets BSDINSTALL_DISTDIR to a directory in
the new filesystem that it can write fetched distfiles to.  As a result,
BSDINSTALL_DISTSITE was incorrectly set to the scratch space on /mnt for
the call to distfetch when grabbing local distfiles, and it would
subsequently fail.

Switch to using the copy of BSDINSTALL_DISTDIR that we stashed off
coming into fetchmissingdists; this one is in-fact set to the path where
the local distfiles are stored.

Patch suggested by jrtc27.

Reported and tested by: Daniel O'Connor <darius dons net au>
MFC after:	1 week

usr.sbin/bsdinstall/scripts/fetchmissingdists

@@ -88,7 +88,7 @@ fi
 if [ ! -z "$LOCAL_DISTRIBUTIONS" ]; then
 	# Copy local stuff first
 	env DISTRIBUTIONS="$LOCAL_DISTRIBUTIONS" \
-		BSDINSTALL_DISTSITE="file://$BSDINSTALL_DISTDIR" \
+		BSDINSTALL_DISTSITE="file://$BSDINSTALL_DISTDIR_ORIG" \
 		bsdinstall distfetch || \
 		error "Failed to fetch distribution from local media"
 fi