[CI][Dev] Report Extra jobs should only run on scheduled jobs

[raulcd]

Describe the bug, including details regarding any error messages, version, and platform.

With the merge of:

• GH-50133: [CI] Make extra labels entirely manual on PRs #50256

We seem to have broken the check to send reports only for nightly runs and we've started receiving on Zulip reports for runs from main:

Report from this run on main:

• https://github.com/apache/arrow/actions/runs/28428004796

Component(s)

Continuous Integration, Developer Tools