Add a "shell" command to unpackers?

[remram44]

To get an interactive shell in the environment?

[remram44]

After comses/reproducible-model-package-examples#1 I am now using busybox binaries that use the internal commands for ls, ln, mkdir, ... making it possible to run a shell by just executing /busybox ash.

This should probably be exposed as an unpacker command.