Set checkout fetch depth for codecov

[ferdnyc]

Honestly thought I'd already committed this, but I guess I was thinking of #636.

When used with Github Actions and its predefined actions/checkout step, Codecov needs the depth overridden in the default git checkout --depth=1 command, which can be done by explicitly setting a fetch-depth option for actions/checkout. This PR changes that step to:

    steps:
    - uses: actions/checkout@v2
      with:
        fetch-depth: 0

See https://community.codecov.io/t/issue-detecting-commit-sha-please-run-actions-checkout-with-fetch-depth-1-or-set-to-0/2571