Inherit breaking flag of sub-PRs in batch-mirror Noir PRs

[Savio-Sou]

Context

Noir changes happening in this repository are currently mirrored in batches to the Noir repository (e.g. noir-lang/noir#4657).

Problem

Sub-PRs within the batches could be breaking PRs (i.e. backward incompatible) containing breaking changes.

But their breaking flags don't bubble up to the batch PR itself, makes spotting breaking changes in the Noir repo less trivial than it should be.

Happy case

When ≥1 sub-PR is breaking, batch-mirror PR is flipped breaking (i.e. adds ! in title).

[Savio-Sou]

Commit overrides in the sync PR bodies sufficiently communicate individual breaking PRs.