Jonathan/debug remote failed builds

Description

This MR makes build shells use the cached buildtree when staging sources, if they exist.

Changes proposed in this merge request:

• Stage the cached buildtree of a source if it exists
• Interactive failure shells don't pass a sysroot in, relying on default behaviour instead.
• Interactive failure shells always offer a shell, rather than requiring the error to include a sysroot.
• bst shell --sysroot takes a bare directory, not one that has a 'root' and 'scratch' subdir.

This merge request is part of #539 (closed), but further discussion is required before the issue can be closed.

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