setenv(var, @optional new_value)
Change the binding of the environment variable var to new_value which should be a string.
If new_value is not specified, the binding is removed. Return void.
getenv("logs_folder", "./")
// ./
setenv("logs_folder", "/logs/")
getenv("logs_folder", "./")
// /logs/
setenv("logs_folder")
getenv("logs_folder", "./")
// ./