which -s only exists on BSD's version of `which`

The `setup.sh` script in this repo does not work on Ubuntu, because the `-s` flag for `which` is only present on the BSD `which` (the one Darwin/macOS ships with), not the one Ubuntu ships with.

This means the script fails as so on Ubuntu:

```
Illegal option -s
Usage: /usr/bin/which [-a] args
Illegal option -s
Usage: /usr/bin/which [-a] args
Error: Neither quicktype nor npx found. Please install quicktype
via brew:

brew install quicktype

or via npm:

npm install -g quicktype
```

A more portable approach, for POSIX systems, is to use `command`. But I will refrain from opening a PR, because I suspect this issue may be considered a wont-fix, given lack of activity in the last year.