Codecov upload issues

[benjeffery]

I've recently fixed the codecov path issues, but we're still seeing some upload problems from forks. See https://github.com/tskit-dev/tskit/actions/runs/8722766656/job/23929376280?pr=2915
This seems to be because forks of public repos don't get the repo secrets, so codecov has to do a tokenless upload which is rate-limited.
There is some redundancy in our uploads, so most of the time we'll get enough uploads in to make the coverage report work.
Ongoing discussion is here: codecov/feedback#301

[benjeffery]

codecov/feedback#273 (comment) suggests that installing the codecov app on a contributor fork may help.