set github.head_ref as env (#493)

Small change to CI.

Author: sbSteveK

.github/workflows/ci.yml

@@ -12,6 +12,7 @@ env:
   BUILDER_HOST: https://d19elf31gohf1l.cloudfront.net
   PACKAGE_NAME: aws-iot-device-sdk-cpp-v2
   LINUX_BASE_IMAGE: ubuntu-18-x64
+  HEAD_REF: ${{ github.head_ref }}
   RUN: ${{ github.run_id }}-${{ github.run_number }}
   AWS_DEFAULT_REGION: us-east-1
   DA_TOPIC: test/da
@@ -84,7 +85,7 @@ jobs:
         run: |
           aws s3 cp s3://aws-crt-test-stuff/ci/${{ env.BUILDER_VERSION }}/linux-container-ci.sh ./linux-container-ci.sh && chmod a+x ./linux-container-ci.sh
           ./linux-container-ci.sh ${{ env.BUILDER_VERSION }} aws-crt-${{ matrix.image }} build -p ${{ env.PACKAGE_NAME }}
- 
+
 
   linux-compiler-compat:
     runs-on: ubuntu-latest
@@ -379,7 +380,7 @@ jobs:
       - name: Build ${{ env.PACKAGE_NAME }} + consumers
         run: |
           echo "Downloading source"
-          git clone --recursive https://github.com/aws/aws-iot-device-sdk-cpp-v2.git --branch ${{ github.head_ref || github.ref_name }}
+          git clone --recursive https://github.com/aws/aws-iot-device-sdk-cpp-v2.git --branch ${{ env.HEAD_REF || github.ref_name }}
           echo "Running builder"
           python -c "from urllib.request import urlretrieve; urlretrieve('${{ env.BUILDER_HOST }}/${{ env.BUILDER_SOURCE }}/${{ env.BUILDER_VERSION }}/builder.pyz?run=${{ env.RUN }}', 'builder.pyz')"
           python builder.pyz build -p ${{ env.PACKAGE_NAME }}