Move the temporary staging directory to artifactdir

Description

It's advantageous when the temporary staging directory is on the same filing system as the artifact cache - files can then be linked or moved to their destination without crossing a filing system. Moving the temporary directory to artifactdir will increase the chances of this.

Changes proposed in this merge request:

• Adjust temp_staging_directory in element.py / _stage_sources_at to use the context's artifactdir as a base.

Not a complete solution, but part of the solution to #684.

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