[Suggestion] Add nushell support as an exception just like ash in distrobox init

**Is your feature request related to a problem? Please describe.**
I'm a nushell user, when creating a distrobox, most distro's package for nushell is called `nushell` not `nu` (the name that `basename` retrieves from `$SHELL`)

**Describe the solution you'd like**
Check if the basename is `nu` then try downloading both `nu` or `nushell` or even symlink the `nu` binary from the host since it uses static linking

**Describe alternatives you've considered**

**Additional context**
Some kind of checking is done when using `ash` i think it won't be too much more code to add this special treatment for `nushell`