CI: Add CODECOV_TOKEN secret (#569)

Starting from codecov-action@v4, Codecov requires setting up a CODECOV_TOKEN, otherwise the upload will fail. Note that forks don't require this token, only PRs from the original repo so forks don't need (and don't have) access to this token.

The global token is generated for aiidalab org at https://app.codecov.io/account/gh/aiidalab/org-upload-token

.github/workflows/ci.yml

@@ -146,3 +146,4 @@ jobs:
               uses: codecov/codecov-action@v4
               with:
                   flags: python-${{ matrix.python-version }}
+                  token: ${{ secrets.CODECOV_TOKEN }}