Is your feature request related to a problem? Please describe.

When using the toolbox enter command to launch the shell in a container, it currently directly uses the $SHELL variable. This can lead to a problem when it can't find shell in container in certain environments, particularly in NixOS host where $SHELL is usually formatted as /run/current-system/sw/bin/<shell-name>. (or something like /usr/local/bin/<shell-name>, ~/.local/bin/<shell-name>)

Describe the solution you'd like

Get shell name from $SHELL and run shell via /usr/bin/env <shell-name>.

Describe alternatives you've considered

Running toolbox enter with SHELL=/usr/bin/fish.