appendToPATH method
Appends newPath to the list of paths in the
PATH environment variable.
If newPath is already in PATH no action is taken.
Changing the PATH has no affect on the parent process (shell) that launched this script.
Changing the path affects the current script and any children that it spawns.
Implementation
void appendToPATH(String newPath) {
  if (!isOnPATH(newPath)) {
    final path = PATH..add(newPath);
    _setEnv('PATH', path.join(delimiterForPATH));
  }
}