Simplify `bst shell` implementation

• Drop support for bst shell --use-buildtree=ask: Proposal on ML
• Drop support for bst shell --use-buildtree=try: As per ML discussion
• Make bst shell --use-buildtree imply --build
• Use single scheduler run for pulling dependencies and buildtree
• Deduplicate cache checks and corresponding warning/error messages with and without pulling enabled

This is in preparation of support for remote execution without downloading blobs.