Simplify base-ref fetch

refspec defaults to refs/heads.
And since this is a throwaway clone, we can safely create a local branch
matching the remote name without concern for collision.

.github/workflows/lint-checksums.yml

@@ -8,7 +8,6 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
-    - name: Fetch base_ref HEAD
-      run: git fetch --depth=1 origin +refs/heads/${{github.base_ref}}:refs/remotes/origin/${{github.base_ref}}
+    - run: git fetch --depth=1 origin +${{github.base_ref}}
     - run: npm ci
     - run: npm run lint:checksums -- origin/${{github.base_ref}}