[GR-66789] Move downstream tests to GitHub actions

[graalvmbot]

• Move downstream tests from our CI dailies to GitHub weeklies (Moday 0:00). They use EA builds and the idea is that rota would check them.
• Enable darwin for all of them
• Add downstream test for pyo3. And disable it because it fails, we regressed again 🤦‍♂️. I'll enable it once I have a fix
• Move the downstream test code out of mx (I didn't want to deal with mx in the action)

Here's a test run: https://github.com/msimacek/graalpython/actions/runs/16028074687/job/45221124251?pr=1