ci: upgrade to actions/checkout@v2

In particular, this appears to fix an extremely annoying bug that was
causing PR builds to fail if they were re-run.

For more details:
https://github.com/actions/checkout/issues/23#issuecomment-572688577

.github/workflows/release.yml

@@ -108,7 +108,7 @@ jobs:
 
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v1
+      uses: actions/checkout@v2
       with:
         fetch-depth: 1