Fix Windows `Command` search path bug
Currently `Command::new` on Windows works differently depending on whether any environment variable is set. For example,
```rust
// Searches for "myapp" in the application and system paths first (aka Windows native behaviour).
Command::new("myapp").spawn();
// Search for "myapp" in `PATH` first
Command::new("myapp").env("a", "b").spawn();
```
This is a bug because the search path should only change if `PATH` is changed for the child (i.e. `.env("PATH", "...")`).
This was discussed in a libs-api meeting where the exact semantics of `Command::new` was not decided but there seemed to be broad agreement that this particular thing is just a bug that can be fixed.
r? libs-api
4606610b21