This PR makes two changes to pr-checks.yml:

1. It limits the push triggers to just the main and release branches. That aligns it with other PR checks. We used to need overlapping push and pull_request triggers, but I am not sure that we still do. In any case, the pr-checks.yml workflow currently runs twice for PRs, which can be misleading and is unnecssary.
2. The other-checks and post-repo-size-comment jobs have some conditions which skip them / steps for Dependabot PRs and PRs from forks. The fetch-base step has a condition like this for Dependabot, even though the overall other-checks job already has a check. Both it and post-repo-size-comment use the PR author for this check, even though the relevant one should be on triggering_actor. This PR removes the unnecessary condition from one and changes the other to use triggering_actor.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

Environments:

• Testing/None - This change does not impact any CodeQL workflows in production.

How did/will you validate this change?

• None - I am not validating these changes.

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Development/testing only - This change cannot cause any failures in production.

How will you know if something goes wrong after this change is released?

• Other - Please provide details.

Are there any special considerations for merging or releasing this change?

• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.