CI: Don't run duplicate jobs for same-repo pull requests

Pushes to branches in this repo already run CI, so run pull_request jobs only
for PRs from forks, which don't trigger push runs here. The push run's checks
still appear on the PR, since checks are attached to the head commit. (This
skips merge-commit testing for same-repo PRs -- the push run tests the branch
as-is.)

.github/workflows/ci.yml

@@ -7,6 +7,10 @@ jobs:
   test:
     name: Test (shard ${{ matrix.shard }})
     runs-on: ubuntu-latest
+    # Same-repo PRs are covered by the push run, so run pull_request jobs only for forks
+    if: >
+      github.event_name != 'pull_request'
+      || github.event.pull_request.head.repo.full_name != github.repository
     strategy:
       matrix:
         shard: [1, 2, 3, 4]
@@ -83,6 +87,9 @@ jobs:
   utilities:
     name: Utilities Tests
     runs-on: ubuntu-latest
+    if: >
+      github.event_name != 'pull_request'
+      || github.event.pull_request.head.repo.full_name != github.repository
     steps:
       - uses: actions/checkout@v4
         with: