ci: fix codecov setup

[xushiyan]

Add codecov upload step after unit test.

Fixes #6

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@a6fbeaa). Click here to learn what that means.

❗ Current head 4023c36 differs from pull request most recent head 08fc553. Consider uploading reports for the commit 08fc553 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #20   +/-   ##
=======================================
  Coverage        ?   68.95%           
=======================================
  Files           ?       11           
  Lines           ?      248           
  Branches        ?        0           
=======================================
  Hits            ?      171           
  Misses          ?       77           
  Partials        ?        0           

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