Disable codecov if PR head and base are different or CODECOV_TOKEN not set

[AndreasLrx]

Description

The CI should not fail anymore when running PR from forks toward this repository, the codecov action will be skipped.
It would have been better to always run but doesn't seem possible for security reason (codecov/codecov-action#29)

It should still work on fork if they set their own CODECOV_TOKEN secret.

Close #169

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update

[optional] Minor fixes

Added/updated tests?

• New tests written
• Existing tests updated
• Tests are not required because this is a CI update
• I need help with writing tests

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.93%. Comparing base (2706640) to head (cc9d4fe).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #174   +/-   ##
=======================================
  Coverage   81.93%   81.93%           
=======================================
  Files          64       64           
  Lines        1129     1129           
  Branches      352      352           
=======================================
  Hits          925      925           
  Misses        144      144           
  Partials       60       60           

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