ROX-12735: add workflow for adding new PRs to test project board

[tommartensen]

Tested in private repositories.
When this has landed in master in this repo, I will:

• open a PR from this repo to check that it IS NOT ADDED to the board
• open a PR from a fork to check that it IS ADDED to the board

Finally, I will submit another PR to set the project URL to the real OSS Triaging board.

[Maddosaurus]

LGTM!
Thanks for renaming the original workflow, I forgot about that 😄
Question: Do we actually add a label to an external PR or is that only the steps description?
A label would be a great idea to be able to filter for all external PRs across all org repos!

[tommartensen]

e932fd6 - I wasn't sure if it was useful to add a label, but that would be it.

[Maddosaurus]

@tommartensen Having a unified label across all repos of the stackrox org makes for a very convenient search query.
And as we want to make it as low friction as possible for our team to review PRs, having a clickable link that just lists all external PRs across all repos sounds really low friction.
So: Thank you very much for adding the label (back) in. That's awesome!

[vikin91]

Looks good and simple - let's see it in action! Many thanks for your help!