Fixes: 825963b5 ("element: Make "--sysroot" take a bare directory")

element.py: Pass bare_directory also to SandboxRemote()

See merge request !928

Issue happens with sourceware.org server.

Fixes #742

Use less fetchers in overnight build to work-around limitations from servers.

Closes #742

See merge request !929

Get the artifact cache from the context instead.

The artifact cache backend does not affect build outputs and we anyway
no longer have pluggable artifact cache backends. This hardcodes
CASCache instead of removing the entry completely to avoid cache key
changes.

This changes CASCache from a subclass to a delegate object of
ArtifactCache. As the lower layer, CASCache no longer deals with
elements or projects.

Fixes #659.