SetEnv will set the os environment variable for the current program to a given value,
and prepares a cleanup function to restore the original state of the environment variable.
Spec using this helper should be flagged with Spec.HasSideEffect or Spec.Sequential.
UnsetEnv will unset the os environment variable value for the current program,
and prepares a cleanup function to restore the original state of the environment variable.
Spec using this helper should be flagged with Spec.HasSideEffect or Spec.Sequential.