`bst shell --use-buildtree ask` uses `--pull` inconsistently

Background

While reproducing #1144 (closed) I dug a little into bst shell --use-buildtree, because I needed to figure out how the logic worked in general.

When bst shell --use-buildtree ask is specified, and a remote build tree exists, we prompt the user asking whether they want to pull a build tree. In every other case, however, bst shell --use-buildtree will assume the same option as --pull.

This is inconsistent and confusing, since it makes two options overlap through a later prompt.

Task description

We should make bst shell --use-buildtree always obey --pull, or always prompt, but not both.

My preference is the former, with a positive response with a falsy --pull defaulting to not attempting to pull a build tree.

Acceptance Criteria

bst shell shouldn't muddle two options in a prompt anymore.

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