build.rs: fix formatting — heartwood

.radicle/native.yaml runs “cargo fmt –check” and that fails without this change.

Signed-off-by: Lars Wirzenius liw@liw.fi

 use std::process::Command;
 use std::env;
 use std::process::Command;
 fn main() {
     // Set a build-time `GIT_HEAD` env var which includes the commit id;