`toolbox enter`: run shell in container via `/usr/bin/env <shell-name>` instead of using `$SHELL` directly

**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/`. (or something like `/usr/local/bin/`, `~/.local/bin/`)

**Describe the solution you'd like**

Get shell name from `$SHELL` and run shell via `/usr/bin/env `.

**Describe alternatives you've considered**

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

Inspect the implementation of the `toolbox enter` command and trace how it obtains and launches `$SHELL`. Confirm the shell name is extracted and invoked through `/usr/bin/env`; done means shells whose host paths are unavailable inside the container can still start.

Written by the indexing model from the issue text.