Set step_script to be used by default by custom executor
Follow-up for #27957.
The introduced configuration option should be set to be true
by default. With that custom executor should start sending step_script
instead of renaming it to build_script
. However, at user's will, when the configuration option is set to false
explicitly, custom executor should fallback to the legacy `build_script.
Edited by Tomasz Maczukin