Revert ci.sh changes.

The PantsRunIntegrationTest and ci.sh changes belong in
https://rbcommons.com/s/twitter/r/969/, save them for then.

Also add a missing direct dep on coverage.
Fixup PythonTestBuilder env pollution.

Bake the main pytest entrypoint into the test runner pex
so that any pexes run during tests do not have their entrypoints
overridden.