gha: Add checkout step

.github/workflows/purge-readme-image-cache.yml

@@ -10,6 +10,10 @@ jobs:
     runs-on: ubuntu-18.04
     steps:
 
+    - uses: actions/checkout@v1
+      with:
+        fetch-depth: 1
+
     - run: |
         IMAGE_LIST=$(curl -sL https://github.com/${GITHUB_REPOSITORY} | \
           grep -oE '<img src="https?://camo.githubusercontent.com/[^"]+' | \