Update std doctests for android
This updates some doctests that fail to run on android. We will soon be supporting cross-compiled doctests, and the `arm-android` job fails to run these tests.
In summary:
- Android re-exports some traits from linux under a different path.
- Android doesn't seem to have common unix utilities like `true`, `false`, or `whoami`, so these are disabled.
Matthias Krüger
036607ebba