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>

(cherry picked from commit 12b92f3ed82aa71c0eba246ce3053ef225724570)