another bugfix in github event pr base/head sha

Author: trevorcampbell

.github/workflows/update_build_environment.yml

@@ -21,10 +21,10 @@ jobs:
         echo "Checking if Dockerfile was modified since last commit on this PR"
         echo "GitHub PR action type: ${{ github.event.action }}"
         if [ "${{ github.event.action }}" == "opened" ]; then
-            echo "GitHub base ref: ${{ github.pull_request.base.sha }}"
-            echo "GitHub head ref: ${{ github.pull_request.head.sha }}"
-            BEFORE=${{ github.pull_request.base.sha }}
-            AFTER=${{ github.pull_request.head.sha }}
+            echo "GitHub base ref: ${{ github.event.pull_request.base.sha }}"
+            echo "GitHub head ref: ${{ github.event.pull_request.head.sha }}"
+            BEFORE=${{ github.event.pull_request.base.sha }}
+            AFTER=${{ github.event.pull_request.head.sha }}
         else
             echo "GitHub event before: ${{ github.event.before }}"
             echo "GitHub event after: ${{ github.event.after }}"