[test-infra] Enable Codecov for tokenizers

[hvaara]

Codecov is enabled for a few huggingface libraries (ref https://app.codecov.io/github/huggingface).

Can it be enabled for the tokenizers library?

I'm happy to ship a PR with the config changes to the Actions workflow, but admin access is needed to configure the codecov repo secret (ref https://app.codecov.io/github/huggingface/tokenizers/new).