`bst shell --build` should let the user choose whether to use cached build trees

Background

This is technical debt: when !869 (merged) is merged, bst shell --build will always stage the cached build tree if it's found. This is making assumptions about the intent of the user.

Task description

• Add a command-line option to bst shell to stage the cached build tree.
• If the option is not specified, and in interactive mode, prompt the user to choose whether to stage the cached build tree (default to not stage it)

Acceptance Criteria

The user is able to choose whether build artifacts are staged when using bst shell --build

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This issue is related to #311, as both involve deciding whether to use a cached build.