Fix Welcome New Contributors workflow

[swissspidy]

reusable-welcome-new-contributors.yml uses actions/first-interaction to identify newcomers. But somehow that newcomer check doesn't seem to work. It always posts the comment, even for myself 😅

It should really only be posted if it's the user's very first pull request.

So let's add our own workflow step to add such a check, in order to reduce the noise.

Related:

• Every contributor identified as new contributor in repo without issues actions/first-interaction#369