set codecov to not fail_ci_if_error

Codecov keeps failing to upload the coverage report seemingly at random, e.g.

https://github.com/fonttools/fonttools/actions/runs/3686952402/jobs/6240048994#step:8:40

I thought I had already fixed it by setting a CODECOV_TOKEN in the repo secrets with #2763 

but it still keeps pestering us so I think the only option left is to not have the PR checks fail with codecov errors.
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