Fix build-time ensurepip

[yan12125]

It's broken by a CPython bug, which attempts to install pip on the host even for cross-builds.

[yan12125]

Current workaround: run ensurepip on the phone. Enter an adb shell and run:

$ python3.7m -m ensurepip
$ python3.7m -m pip install --upgrade pip setuptools  # This is optional

[yan12125]

Also, latest pip hang after running most commands. Android doesn't allow non-root processes to create hard links, which breaks the lockfile module. See pypa/pip#3532.

Update: pip is moving away from pylockfile: pypa/pip#4766

[yan12125]

Also related to pypa/pip#3063. Possible fixes at https://github.com/yan12125/pip/tree/pip-3063 and https://github.com/yan12125/setuptools/tree/pip-3063