chore: use the fakeenv crate for testing environment variable specific functionality

[haraldmaida]

Testing code that makes decisions based on the value of environment variables faces two problems:

1. The tests can not run in parallel as setting or removing an environment variable in one testcase may influence other testcases
2. As we know from Rust edition 2024 the functions std::env::set_var and std::env::remove_var are unsafe on some operating systems

For the first problem we used the serial_test crate to mark all tests that shall not run in parallel.

Using a fake "environment", that mimics the related functions from the std::env module, for testing solves both problems. The crate serial_test is no longer needed as a dependency.

Refactored tests in colored::tests module to use a fake environment and removed the serial_test dependency.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.70%. Comparing base (29f1efa) to head (9b3e223).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #50   +/-   ##
=======================================
  Coverage   96.69%   96.70%           
=======================================
  Files          30       31    +1     
  Lines        3416     3425    +9     
  Branches     3416     3425    +9     
=======================================
+ Hits         3303     3312    +9     
  Misses        109      109           
  Partials        4        4           

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