ci: fix codecov action

[fmartingr]

• Specify secrets on the flow manually. (Thanks https://github.com/napari/napari/pull/6701/files)
• Removes coverage.txt from the repository
• git-Ignores /coverage.* from the root folder

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 34.08%. Comparing base (595cb45) to head (6cdab88). Report is 34 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #928 +/- ## ========================================== + Coverage 25.58% 34.08% +8.49% ========================================== Files 47 60 +13 Lines 5628 4008 -1620 ========================================== - Hits 1440 1366 -74 + Misses 3989 2420 -1569 - Partials 199 222 +23 ```

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