lbmk script: always clean up /tmp files

export TMPDIR to scripts, and handle it in a way that
we know lbmk set it

delete it at the end of the parent process, but not child
processes; when the lbmk script calls itself, child processes
will not delete the tmp directory.

some scripts in lbmk weren't cleaning up the tmpfiles they
made, and they still don't, but this mitigates that.

now in follow-up commits, i can start cleaning up those
scripts too.

not handled by this patch:
if the user cancels lbmk (ctrl+c), the tmp directory will
still be there. this too will be handled, in subsequent
patches

Signed-off-by: Leah Rowe <leah@libreboot.org>

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