Improve PR job matrix computation

[ofek]

Motivation

I was doing this for the agent repo and I realized that computing the diff requires much less information.

Notes

The step to fetch what is required is now between 5-10 seconds faster. Since this is the job that produces the matrix, all PR jobs now start that much faster.

3 seconds (this PR) https://github.com/DataDog/integrations-core/actions/runs/17678163011/job/50245466046

Recent PR runs:

13 seconds https://github.com/DataDog/integrations-core/actions/runs/17677260476/job/50242378787
8 seconds https://github.com/DataDog/integrations-core/actions/runs/17676395135/job/50239460842
11 seconds https://github.com/DataDog/integrations-core/actions/runs/17670175603/job/50219972764

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.01%. Comparing base (7613a6e) to head (b129a08).
⚠️ Report is 17 commits behind head on master.

Additional details and impacted files

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.