Enable rebuilding PRs with conflicts

[henrymercer]

Since we check in our compiled code, we frequently get merge conflicts in the compiled code. We have a rebuild Action, but label triggers don't run when there are merge conflicts. So this PR adds a workflow_dispatch trigger that lets us rebuild a PR with conflicts via workflow_dispatch.

Example run: https://github.com/github/codeql-action/actions/runs/16809277853

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[Copilot]

Pull Request Overview

This PR enables rebuilding PRs with merge conflicts by adding a workflow_dispatch trigger to the rebuild workflow. Since label triggers don't work when PRs have conflicts, this provides an alternative way to rebuild the compiled code through manual workflow dispatch.

• Added workflow_dispatch trigger alongside existing label trigger
• Updated condition logic to handle both trigger types with fallback values
• Moved PR-specific operations (label removal, commenting) to be conditional on pull request events

Comments suppressed due to low confidence (2)

.github/workflows/rebuild.yml:80

• The set-output command is deprecated and will be removed. Use echo "changes=true" >> $GITHUB_OUTPUT instead.

            echo "::set-output name=changes::true"

.github/workflows/rebuild.yml:92

• [nitpick] The notification logic has been moved outside the conditional block but still executes the same commands. This creates duplicate logic that was previously inside the 'if' statement. Consider keeping this logic together with the push operation or clearly separating the concerns.

          echo "Pushed a commit to rebuild the Action." \
            "Please mark the PR as ready for review to trigger PR checks." |
            gh pr comment --body-file - --repo github/codeql-action "$PR_NUMBER"
          gh pr ready --undo --repo github/codeql-action "$PR_NUMBER"