os/mac: fix types

`@mdfind` needs to store `T::Array[String]` to have correct return type.

Also cast `String#scan` result as the returned type is based on whether
input has capture groups, which Sorbet cannot handle. Currently, Sorbet
uses more general `T::Array[T.any(String, T::Array[String])]` return. We
could alternatively run `.flatten!` but this adds more overhead when
Sorbet is disabled for users than just a no-op function call to `T.cast`