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
12b92f3ed8