Try "fetch-depth: 0" on the extra checkouts

.github/workflows/pr-checks.yml

@@ -24,10 +24,12 @@ jobs:
       uses: actions/checkout@v2
       with:
         ref: ${{github.base_ref}}
+        fetch-depth: 0
     - name: Checkout head of PR
       uses: actions/checkout@v2
       with:
         ref: ${{github.head_ref}}
+        fetch-depth: 0
 
     - name: Set up Python 3.7
       uses: actions/setup-python@v2