Try to fix coverage collection from nix

.circleci/config.yml

@@ -147,7 +147,7 @@ jobs:
           name: "Report Coverage"
           command: |
             nix-shell -p 'python.withPackages (ps: [ ps.codecov ])' --run \
-              'codecov --file ./result-doc/share/doc/*/.coverage'
+              'cp ./result-doc/share/doc/*/.coverage ./; coverage combine; codecov --file .coverage'
 
 workflows:
   version: 2

.coveragerc

@@ -3,4 +3,9 @@ source =
     _zkapauthorizer
     twisted.plugins.zkapauthorizer
 
+[paths]
+source =
+    src/
+    /nix/store/*/lib/python*/site-packages/
+
 branch = True