Ensure equivs-build places .deb in right dir when $TMPDIR set

When $TMPDIR is set, equivs-build will store the .deb there instead
of the current directory, but find-dbgsym-packages is expecting the
.deb will be placed in the temporary directory that it created.
This produces the following error with $TMPDIR and --deb:

BUG: Not exactly one .deb file generated at /usr/bin/find-dbgsym-packages line 466.