element.py: Cleanup temporary staging directories on termination

Tristan Van Berkomrequested to merge
tristan/cleanup-staging-tempdir into master

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.

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