Fix clippy error

Author: Jon Gjengset

build.rs

@@ -69,14 +69,15 @@ fn compile_probe() -> Option<ExitStatus> {
     fs::write(&probefile, PROBE).ok()?;
 
     // Make sure to pick up Cargo rustc configuration.
-    let mut cmd = if let Some(wrapper) = env::var_os("CARGO_RUSTC_WRAPPER") {
-        let mut cmd = Command::new(wrapper);
-        // The wrapper's first argument should always be the path to rustc.
-        cmd.arg(rustc);
-        cmd
-    } else {
-        Command::new(rustc)
-    };
+    let mut cmd = env::var_os("CARGO_RUSTC_WRAPPER").map_or_else(
+        || Command::new(&rustc),
+        |wrapper| {
+            let mut cmd = Command::new(wrapper);
+            // The wrapper's first argument should always be the path to rustc.
+            cmd.arg(&rustc);
+            cmd
+        },
+    );
 
     cmd.stderr(Stdio::null())
         .arg("--edition=2018")