Fix codecov integration

[devmotion]

#188 broke codecov integration: Version 4 of the action does not support tokenless iploads anymore. Thus currently codecov uploads will always fail, see eg https://github.com/kalmarek/Arblib.jl/actions/runs/8724232667/job/23934347761#step:8:34. This error was masked by the fail_ci_if_error: false, so you might want to consider changing it to true (at the cost of spurious test failures sometimes since codecov servers are not completely reliable in my experience).

To fix codecov integration, in addition to this PR you have to go to the codecov webpage, navigate to the settings for this repo, and copy the token shown there to a Github repo secret named CODECOV_TOKEN.

[kalmarek]

@devmotion Thanks for detailed info and instructions!

I've added the token to the repo. I'd personally change fail_ci_if_error to true, but let @Joel-Dahne voice his opinion on this;)

[Joel-Dahne]

Changing fail_ci_if_error to true sounds reasonable to me as well!

[kalmarek]

@devmotion and now macOS fails as it tries to use (the non-existent) token from your fork? the other ones just resolve to tokenless upload. Is this ok to be merged?

EDIT: seems that the coverage was updated on codecov successfully!

[devmotion]

An alternative to true could be something like JuliaStats/StatsBase.jl#920 (which was motivated by test failures in PRs from forks that we saw in StatsBase).

[kalmarek]

I'm fine with simple true; the more complicated it is the more maintenance burden it becomes