xtask: don't drop cargo exec iterator in run()

xtask/src/main.rs

@@ -228,7 +228,7 @@ fn run_cli(
         .position(|arg| arg == "--")
         .map(|extra_args_index| args[extra_args_index..].to_vec());
 
-    for _ in cargo_exec(
+    let mut i = cargo_exec(
         "run",
         "cli",
         triple,
@@ -236,7 +236,8 @@ fn run_cli(
         debug,
         additional_flags,
         true,
-    )? {}
+    )?;
+    while let Some(Ok(_)) = i.next() {}
     Ok(())
 }