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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.44%. Comparing base (62b67a3) to head (9c70ad8).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #553      +/-   ##
==========================================
+ Coverage   95.90%   97.44%   +1.54%     
==========================================
  Files          14       14              
  Lines        2686     2705      +19     
==========================================
+ Hits         2576     2636      +60     
+ Misses        110       69      -41     

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[zimeg]

🗣️ note: The requests to run tests above are an artifact of our current setup when running workflows in the "staging" environment. I'll approve these now, but we might remove this check if these changes are alright!