Use Platform.artifact_cache() instead of reconstructing CASCache from context

Background

Currently _sandboxremote.py creates is own CASCache object using the sandbox's context object:

self.cascache = CASCache(self._get_context())

Having two instances may be dangerous, and the 'main' cascache should be available via Platform.artifact_cache(), so we should use that.

Task description

• Determine if it's possible to use Platform.artifact_cache()
• Change sandboxremote to use it.

Acceptance Criteria

Should be identical functionality to before the change, possibly improved. Remote execution jobs should work as before.

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