Add "build_cmd_once"

[igor-petruk]

This will run only during the first build, before build_cmd. If we are rebuilding the binary, this won't run again. We can add "scriptisto cache invalidate" command to force it to rerun.

Use cases:

• Setup development environment, like virtualenv+pip for Python or "dotnet restore"
• Fetch extra files from the web via curl
• Prebuild something heavy that is rarely to change

[igor-petruk]

Fixed in v0.4.2

[igor-petruk]

build_once_cmd