Fix slack notifications with external PRs

[sjohnr]

See Gamesight/slack-workflow-status#38

[sjohnr]

This fix was unsuccessful. Reopening to investigate further.

[sjohnr]

Upon investigation, the existing fix put in place should have resolved the issue.

Notes:

The current step looks like this:

      - name: Send Slack message
        # Workaround while waiting for Gamesight/slack-workflow-status#38 to be fixed
        # See https://github.com/Gamesight/slack-workflow-status/issues/38
        uses: sjohnr/slack-workflow-status@9eb3eb361c03f8bb9271782f987f472adfc31056
        with:
          repo_token: ${{ secrets.GITHUB_TOKEN }}
          slack_webhook_url: ${{ secrets.SLACK_WEBHOOK_URL }}
          channel: '#spring-security-ci'
          name: 'CI Notifier'

This step points to this commit in a fork of the slack-workflow-status action, but GitHub doesn't seem to be using this commit.

The fix filters out PRs from external forks of the project by comparing the repository url of the pull request to the repository url of the workflow run.

See an example of the workflow run API response the action is working from. The action also queries the jobs for the workflow run.