flowey: implement is_executable for windows

#2620 implements is_executable for unix, but not for windows. There is a `is_executable` [crate](https://crates.io/crates/is_executable) we could use, or we could just pull the code for windows from [here](https://github.com/fitzgen/is_executable/blob/ee6ad10070a9fc4786128e0f3ee5d343b957b84f/src/lib.rs#L58) 