add shell_builder.py
add shell_builder.py

Summary:
`shell_builder.py` allows running the fbcode_builder logic
on the host rather than in a container.

It emits a bash script with `set -exo pipefail` configured such that
any failing step will cause the script to exit with failure.

Refs: https://github.com/facebook/watchman/pull/639

Reviewed By: simpkins

Differential Revision: D9552411

fbshipit-source-id: c7835deedf07ea342dcb3de61d576a4fb5439985

facebook/wangle,facebook/wangle,facebook/wangle