ci: do not trigger tests in main

[knqyf263]

Description

All PRs are merged through the merge queue now. And tests are triggered by the merge_group event and must pass before merging.
https://github.com/aquasecurity/trivy/actions?query=event%3Amerge_group

Triggering tests in the main branch is no longer needed as it is already done with merge_group. Please see the attached screenshot. The same tests are running with merge_group and push.

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).