Refactor Github Action per b/485167538 (#14967)

This is a http://go/LSC run by http://go/ghss to automatically refactor
your Github Actions per http://b/485167538.

This is a PR to help you upgrade to the latest standards in Github
Actions.

Please merge this PR to accept the changes. NOTE: if you do not accept
this PR, it may be force merged by the GHSS team. See http://b/485167538
for more details.

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

Author: google-admin

.github/workflows/check_base_os.yml

@@ -35,7 +35,7 @@ jobs:
         run: |
           # Get the list of changed files compared to the target branch
           # and filter for unique directories under 'projects/'.
-          CHANGED_DIRS=$(git diff --name-only ${{ github.base_ref }} ${{ github.head_ref }} | \
+          CHANGED_DIRS=$(git diff --name-only ${GITHUB_BASE_REF} ${GITHUB_HEAD_REF} | \
             grep '^projects/' | \
             xargs -n 1 dirname | \
             sort -u)
@@ -53,9 +53,11 @@ jobs:
         if: steps.changed-projects.outputs.changed_dirs != ''
         run: |
           EXIT_CODE=0
-          for project_dir in ${{ steps.changed-projects.outputs.changed_dirs }};
+          for project_dir in ${STEPS_CHANGED_PROJECTS_OUTPUTS_CHANGED_DIRS};
           do
             echo "--- Checking ${project_dir} ---"
             python3 infra/ci/check_base_os.py "${project_dir}" || EXIT_CODE=$?
           done
           exit $EXIT_CODE
+        env:
+          STEPS_CHANGED_PROJECTS_OUTPUTS_CHANGED_DIRS: ${{ steps.changed-projects.outputs.changed_dirs }}