Use CODECOV_TOKEN secret

The new version of Codecov seems to require tokens be set even for
public repositories; the secret was taken from the codecov website[1]
and added as a repository secret[2].  It isn't clear if this is intended
as required in the long term[3] but for now it appears to be.

[1] https://app.codecov.io/gh/PhilanthropyDataCommons/service/settings
[2] https://github.com/PhilanthropyDataCommons/service/settings/secrets/actions
[3] codecov/codecov-action#1035

.github/workflows/test.yml

@@ -58,3 +58,5 @@ jobs:
 
       - name: Upload test coverage
         uses: codecov/codecov-action@v4
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}