Use fetch-depth 0 for the nightly-check checkout

actions/checkout doesn't fetch tags by default, so we have no way of knowing if HEAD points at the latest 'nightly' tag.

.github/workflows/nightly.yml

@@ -15,6 +15,8 @@ jobs:
     steps:
     - uses: actions/checkout@v2
       name: checkout
+      with:
+        fetch-depth: 0
     
     - name: nightly-check
       id: check