Use utils._tempdir() which in turn uses _signals.terminator() for
this purpose.

This also changes utils._tempdir() to use utils._force_rmtree()
so that it is safe to use for cleaning up the staging directory.

This patch fixes orphaned temporary directories being left behind
in the artifact cache directory at forceful termination time.

element.py: Cleanup temporary staging directories on termination

See merge request !1080