Fix #182 by explicitly checking out pull_request.head.sha (#184)

Similar to actions/checkout#518

Author: advaitjain

.github/workflows/xtensa.yml

@@ -32,6 +32,8 @@ jobs:
     name: Fusion F1 Unit Tests (presubmit)
     steps:
       - uses: actions/checkout@v2
+        with:
+          ref: ${{ github.event.pull_request.head.sha }}
       - run: |
           echo ${{ secrets.TFLM_BOT_PACKAGE_READ_TOKEN }} | docker login ghcr.io -u tflm-bot --password-stdin
           docker run --rm -v `pwd`:/opt/tflite-micro ghcr.io/tflm-bot/xtensa:latest /opt/tflite-micro/tensorflow/lite/micro/tools/ci_build/test_xtensa_fusion_f1.sh