ikiwiki-mass-rebuild: under $! before setting $) to avoid strange errno issue

ikiwiki-mass-rebuild: under $! before setting $) to avoid strange errno issue

This fixes a problem sgran saw on alioth. Apparently nss-db sets errno to
ENOENT as a side effect trying to read an optional file, but succeeds
anyway. Then, somehow, errno remains set across the library calls made by
$).

So unset it first as a workaround; there's probably a nss-db, libc, and/or
perl bug underneath.