### What changes were proposed in this pull request?

This PR fixes the notify_test_workflow.yml to detect build_main.yml that is for PR builds.

In addition, this PR fixes the link of build status in README.md.

### Why are the changes needed?

To make the build fixed.

### Does this PR introduce _any_ user-facing change?

No, dev-only.

### How was this patch tested?

N/A. Should better be merged fixed and tested since it's already broken.

Closes #36932 from HyukjinKwon/SPARK-39521-followup.

Authored-by: Hyukjin Kwon <gurwls223@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>