Don't create a build_shell directory when just calling generate
Dont use the output roller for processes that don't prints to tty
Fix env_script builder, so it doesn't unset path

When setting an environment variable, unconditionally save the old value
into a variable.

I don't remember why I ended up storring all the different values from
the different projects. Ideally they should be merged into a list, where
the last defined value comes last