wireit ignores npm's `shell-script` setting

In #648 @aomarks already mentions that the `shell-script` setting isn't supported yet, but there was no explicit issue for it yet.

I use `shell-script=/bin/zsh` for some projects because `**` globs work as expected instead of the default shell (at least on macOS).

For example when I run the following command with npm's default shell `sh` it only matches a subset of the expected match

```sh
> node -e "console.log(process.argv)" src/**/*.test.ts
[ '/path/to/my/project', 'src/test/a.test.ts', 'src/test/b.test.ts' ]
>
```

but there's also a `src/test/sub/a.test.ts` which is not included in `sh`'s expansion.

The actual command I noticed this with is `tsx --test src/**/*.test.ts` which uses `tsx` to run my tests written in TS with node's builtin test runner.