CI: Make nightly jobs "manual" in merge requests

I noticed that the nightly tests ran in MRs, which is not supposed to be the case. Something must have changed in Gitlab.

This MR modifies the rules so they again won't run except as scheduled.

I wanted to make them manual, but gitlab refused the whole CI configuration with this error: https://gitlab.com/gpaw/gpaw/-/pipelines/880038828. Strange!