dump output paths to help debug failure to copy coverage out of nix build

.circleci/report-coverage.sh

 #! /usr/bin/env nix-shell
 #! nix-shell -i bash -p "python.withPackages (ps: [ ps.codecov ])"
+find ./result-doc/share/doc
 cp ./result-doc/share/doc/*/.coverage.* ./
 python -m coverage combine
 python -m coverage report