Be SHELL agnostic

[fenollp]

Right now bash is required only to be able to restore the set of environment variables on each substep.
If at all possible it would be much faster to just start a non-login shell (overridable through YML shell: /bin/sh) with -s and pipe commands in and out of that.
See http:https://technosophos.com/2014/07/11/start-an-interactive-shell-from-within-go.html