Snactor is broken after n/n-1 upgrade path introduction

[fernflower]

More details in oamg/leapp-repository#864 (comment)

Long story short, pretty much all snactor functionality is broken with the following exception raised

leapp.models.fields.ModelViolationError: The value of "target" field is None, but this is not allowed

[fernflower]

The solution is to set target_release and and upgrade_flavor env vars manually before running snactor.
For example, for 8to9 upgrade it will be

export LEAPP_UPGRADE_PATH_TARGET_RELEASE=9.0
export LEAPP_UPGRADE_PATH_FLAVOUR=default

This should be mentioned in the upstream docs as well, once it's done this bug can be closed.