formula: widen type for system args

We can pass Integers here too, for e.g. make flags

```
TypeError: Parameter 'args': Expected type T.any(Pathname, String), got type Integer with value 4
Caller: /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/pypy.rb:68
Definition: /usr/local/Homebrew/Library/Homebrew/formula.rb:1985
```