Update to Brussels 0.2.0

[pietroalbini]

This PR updates Brussels to 0.2.0 (oxidecomputer/brussels#7) and makes the necessary changes to be compatible with it.

This release of Brussels doesn't change the manual steps needed to publish a release (it still only does the tagging and initial build), and instead contains these changes:

• Records the commit we are releasing in the manifest, so that the rest of the (future) release process will be able to rely on it.
• Rather than hardcoding the release groups in the Brussels source code, there is now a file in the Hubris repository, brussels.toml. The fine defines both how to create the tag and the list of boards to release.

Along with the update I changed the GitHub release creation code to use the commit we are actually releasing, instead of releasing the latest commit on main unconditionally. This will prevent cases like the all-sp-v1.0.49 tag pointing to the wrong commit.