Include appropriate craft-parts environment variables when packing with `--shell`, `--debug`, etc.

### What needs to get done

Include the relevant environment variables set by craft-parts when using `--debug` or `--shell` on a lifecycle command.

### Why it needs to get done

testing out override scripts with variables like `$CRAFT_STAGE` already set would make debugging much faster.

Start by locating the lifecycle command handling --debug and --shell, then trace how craft-parts environment variables are currently prepared. The work is done when those commands expose relevant variables such as $CRAFT_STAGE while running override scripts, without changing other lifecycle behavior.

Written by the indexing model from the issue text.