allow build.sh to be used for windows

[rabbah]

It's possible to use bash scripts on windows (see https://altis.com.au/installing-ubuntu-bash-for-windows-10) in which case one could avoid the need for creating build.cmd if it's desirable. We would need a flag though for the cli to allow "sh" instead of "cmd" for the build file lookup for windows.

Should we support this?