Refactor Github Action per b/485167538 (#644)

Co-authored-by: Ben Knutson <benknutson@google.com>

Author: google-admin

.github/workflows/changelog.yml

@@ -24,7 +24,7 @@ jobs:
           # automatically (see https://github.com/webcomponents/polyfills). To
           # diff with the base branch, we need to fetch that too (and we only
           # need its latest commit).
-          git fetch origin ${{ github.base_ref }} --depth=1
+          git fetch origin ${GITHUB_BASE_REF} --depth=1
           if [[ $(git diff --name-only FETCH_HEAD | grep CHANGELOG) ]]
           then
             echo "A CHANGELOG was modified. Looks good!"