bring buildstream up to date with upstream master

Brings buildstream up to date with current buildstream master. It incorporates changes to how junction overlaps are handled, and an important bugfix that was blocking development on the buildstream2 branch of freedesktop-sdk.

This is the image that was used for https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/pipelines/165437040. which passed CI with buildstream 2.