consistent env variables

Author: plowsof

.github/workflows/ci.yml

@@ -73,11 +73,11 @@ jobs:
         run: |
           if [[ "${GITHUB_EVENT_NAME}" == "pull_request" ]]; then
             # diff all files in the PR
-            git fetch origin ${{ github.base_ref }}
-            BASE_SHA=$(git merge-base HEAD origin/${{ github.base_ref }})
+            git fetch origin $GITHUB_BASE_REF
+            BASE_SHA=$(git merge-base HEAD origin/$GITHUB_BASE_REF)
           else
             # diff only the latest commit
-            BASE_SHA=${{ github.event.before }}
+            BASE_SHA=$GITHUB_EVENT_BEFORE
           fi
           echo "BASE_SHA=$BASE_SHA" >> $GITHUB_ENV
           echo "Using BASE_SHA=$BASE_SHA"