because setenv is async, should await

[david-shu]

in file : serious_python_android.dart

`
setenv("PYTHONINSPECT", "1");
setenv("PYTHONDONTWRITEBYTECODE", "1");
setenv("PYTHONNOUSERSITE", "1");
setenv("PYTHONUNBUFFERED", "1");
setenv("LC_CTYPE", "UTF-8");
setenv("PYTHONHOME", pythonLibPath);
setenv("PYTHONPATH", moduleSearchPaths.join(":"));

// set environment variables
if (environmentVariables != null) {
for (var v in environmentVariables.entries) {
setenv(v.key, v.value);
}
}
`
Due to the asynchronous nature of setenv, it may cause libpython loading exceptions or require dart to pass environment variables to python scripts, but setenv is not set in a timely manner, resulting in python script exceptions