Turn off the scheduled run for job that updates googleapis_commitish inside generation_config.yaml. This is to stop nightly generation PRs like #13370 to avoid confusion.
For more context, read doc.

Note this does not turns down hermetic build code generation workflow:

• This workflow that updates googleapis commit for librarian.yaml now also updates generation_config.yaml. So hermetic build code generation is triggered on that PR. Instead of nightly generation PR, a generation PR like chore: update googleapis commitish to ff15be5 #13385 should contain the same contents.
• If needed, manual trigger is still possible.

For googleapis/librarian#6220