Simplify test job checkout ref to github.sha

Consistent with the deploy job; the pull_request_target workaround of
explicitly specifying the PR head SHA is no longer needed now that we
use the pull_request trigger.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: lwr20

.github/workflows/ci.yml

@@ -21,7 +21,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5  # v4
         with:
-          ref: ${{ github.event.pull_request.head.sha || github.sha }}
+          ref: ${{ github.sha }}
       - name: Setup Go ${{ env.GO_VERSION }}
         uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff  # v5
         with: