ci: increase fetch-depth to 2

Codecov apparently has trouble detecting the git commit if the
fetch depth is only 1 (the default). Therefore increase it.
See codecov/codecov-action#190 for
more details.

.github/workflows/tests.yml

@@ -21,6 +21,8 @@ jobs:
     steps:
     - name: "Clone Repository"
       uses: actions/checkout@v2
+      with:
+           fetch-depth: 2  # codecov requires this (https://github.com/codecov/codecov-action/issues/190)
     - name: "Run Tests"
       uses: osbuild/containers/src/actions/privdocker@e4de123f43b95e99dfe8eed0bd5a1cd58db50715
       with: