Github Action for PR needs to run if **source** branch is a forked develop

[roscopeco]

Currently, the build action on PR only runs if the branch is not develop, which is as intended (we want to have a separate build for develop that doesn't upload artifacts but instead creates a snapshot release).

However, the case where the source branch of a PR is a develop in a forked repository also prevents the action running.

So, we need to find a way to make the action be ignore if the target (and only the target) branch is develop in this repo.