CI: Add CODECOV_TOKEN secret

[danielhollas]

Starting from codecov-action@v4, Codecov requires setting up a CODECOV_TOKEN, otherwise the upload will fail. Note that forks don't require this token, only PRs from the original repo so forks don't need (and don't have) access to this token.

I've generated a global token for aiidalab org at https://app.codecov.io/account/gh/aiidalab/org-upload-token

I then I added the token as an organization-wide secret on Github so that we don't have to configure it on every repo.

https://github.com/organizations/aiidalab/settings/secrets/actions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.07%. Comparing base (3bce8e7) to head (b75c762).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #569   +/-   ##
=======================================
  Coverage   87.07%   87.07%           
=======================================
  Files          27       27           
  Lines        4649     4649           
=======================================
  Hits         4048     4048           
  Misses        601      601           

Flag	Coverage Δ
python-3.10	87.07% <ø> (ø)
python-3.9	87.10% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[unkcpz]

Good, thanks for taking care of this.