Shell --build has optional buildtrees

Description

This is based on the issue description in #740 (closed), it allows the user to select whether to use a buildtree when running shell --build on the command line or if not specified but in interactive mode it will prompt the user.

I have done the parts of this feature that are blocking, the non blocking follow on work could be to add configuration options to change the default behaviour or even to attempt to pull a build tree from this command. However these do not seem to be blocking and add to the complexity.

I believe the configuration and pull work would be additional features rather than tech-debt.

This merge request, when approved, will close: #740 (closed)

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