CI: Use fetch-depth: 2 for Codecov

Otherwise it sometimes complains about not being able to determine the SHA.

Author: Geod24

.github/workflows/ci.yml

@@ -14,6 +14,9 @@ jobs:
 
     steps:
     - uses: actions/checkout@v2
+      with:
+        # For codecov
+        fetch-depth: 2
     - uses: dlang-community/setup-dlang@v1
       with:
         compiler: ${{ matrix.dc }}