According to the tempfile(1) manpage:

       -d, --directory DIR
              Place the file in DIR.

... but in mktemp(1):

       -d, --directory
              create a directory, not a file

       [...]

       -p DIR, --tmpdir[=DIR]
              interpret TEMPLATE relative to DIR; if DIR is not specified, use
              $TMPDIR  if set, else /tmp.  With this option, TEMPLATE must not
              be an absolute  name;  unlike  with  -t,  TEMPLATE  may  contain
              slashes, but mktemp creates only the final component

The mktemp usage is surprisingly obtuse, to be honest, but I think
that's about it: -d is now -p. We don't specify a TEMPLATE so the
remark about that shouldn't matter here.

The script is still not "shellcheck-clean" (there's a warning about
the trap), but it should fix the immediate problem, which is:

Closes: #3