tests: Fix environment tests ensuring "enable_service()" is effective
tests: Fix environment tests ensuring "enable_service()" is effective

This commit fixes the following tests:

* test_environment_persist
* test_expand_environment
* test_ci_name_environment_variable

It basically ensures calling "enable_service" remove variable of the
environment being used. Prior to this commit the variable were removed
and the call to "push_env()" was setting back the value from the host.

scikit-build/scikit-ci