Skip integration tests on fork PRs

[ocavue]

Integration tests git push to the repo. Fork PRs only get a read-only GITHUB_TOKEN, so the test job always 403s. This PR skips the test job when the PR comes from a fork.

If my understanding is correct, coverage is unchanged because merge_group still runs the full suite before merge:

name: CI

on:
  pull_request:
  merge_group:
    branches:
      - main

The resulting flow for a fork contribution:

1. A PR from their fork, pull_request fires. Code is untrusted, GITHUB_TOKEN is read-only, CI / Test is skipped.
2. Maintainer reviews, approves, and clicks Add to merge queue button.
3. merge_group fires in the base repo context with a writable GITHUB_TOKEN, running the full suite against the now-trusted code.
   • If all jobs in merge_group pass, the commit lands on main.
   • If CI / Test in merge_group fails, the required check CI / CI OK will also fail, and the commit won't land.

[Andarist]

clever!