Formalize PR process for breaking changes

[thomas-bc]

Originating Project/Creator
Affected Component	CI
Affected Architectures(s)
Related Issue(s)	#2278
Has Unit Tests (y/n)
Builds Without Errors (y/n)
Unit Tests Pass (y/n)
Documentation Included (y/n)

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Change Description

When building external repositories (with reusable-builder.yml) such as our tutorial suite, CI will first check for the existence of the branch pr-<PR_NUMBER> on the external repository. If a branch is found, use it. If not, use devel.

Rationale

This will allow developers to propose changes (and CI to run on those changes) without needing to merge them into devel, which was a source of weirdnesses in the process. Also allows for concurrent PRs to be reviewed, without having to merge one first for the other one to be reviewable.

Comments

This is honestly just a proposal. Everything is interconnected and that makes it hard to come up with a really clean solution. This is the best I could do - let me know if you can think of other ideas.

Future work

Consider dropping the main branch on the tutorial repos, and only use devel like we're doing on core F´ and tools/GDS.