ci: add codecov token

[danielroe]

with v4 of the codecov action, we need to supply a codecov token (pointed out by @ricardogobbosouza). I've added the token in GH variables...

[cloudflare-workers-and-pages]

Deploying nuxt-image with    Cloudflare Pages

Latest commit:	a6aeeec
Status:	✅  Deploy successful!
Preview URL:	https://4c7f194f.nuxt-image.pages.dev
Branch Preview URL:	https://codecov-token.nuxt-image.pages.dev

View logs

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.66%. Comparing base (8701991) to head (a6aeeec).
⚠️ Report is 685 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1342      +/-   ##
==========================================
- Coverage   68.20%   59.66%   -8.54%     
==========================================
  Files          76       78       +2     
  Lines        4299     5080     +781     
  Branches      397      409      +12     
==========================================
+ Hits         2932     3031      +99     
- Misses       1339     2019     +680     
- Partials       28       30       +2     

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