Correct crash after staging tars with read-only directories

Description

Fixes a crash in element.py/_stage_sources_at in which tempfile will break if we stage files into the temporary directory which have read-only, unreadable or unexecutable directories in them. In other words any directory with less than rwx permissions for the owner will break it.

Proposed changes

Changes proposed in this merge request:

• Recursively chmod all directories in the temporary directory before it's deleted.
• Add a test which uses a tarball which would otherwise cause this crash.

Fixes

This merge request, when approved, will close issue/bug: #601 (closed)

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