CI: Label PRs with B2-breaksapi if substrate changes are labelled B2-breaksapi (#862)

* CI: Split common functions into and make...

... check the substrate changes for various
labels. If labelled, will be included in the changelog, otherwise will
silently be left out.

* replace non-standard /bin/bash with /usr/bin/env bash

* CI: make /bin/sh-compatible

* fix newline chars sometimes breaking has_label()

* tag change with 'B2-breaksapi'...

... if any substrate changes are also labelled 'B2-breaksapi'

* Remove debug exits and uncomment labelling

* exit early if we label with breaksapi

* fix
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