ci: run tests without permission checks for origin pull requests

[zimeg]

Summary

This PR runs tests without permission checks for origin pull requests - changes sent from a forked branch still require approval: #552

Preview

The checks of the example above error on forks:

Error: Integration tests from forked branches require maintainer approval.
Notice: Dispatch a test run at https://github.com/slackapi/slack-github-action/actions/workflows/integration.yml with ref 'pull/552/head'

🔗 https://github.com/slackapi/slack-github-action/actions/runs/21665028776/job/62458200754?pr=552#step:2:7

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.