Adding CODECOV_TOKEN in CI

This should remove the error from Codecov in CI.

Author: moufmouf

.github/workflows/tests.yaml

@@ -48,3 +48,5 @@ jobs:
         with:
           files: './coverage.xml'
           fail_ci_if_error: true
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}