Fix deadlock when build scripts are waiting for input on stdin

Author: arlosi

crates/cargo-util/src/process_builder.rs

@@ -355,6 +355,8 @@ impl ProcessBuilder {
             let (mut child, argfile) = spawn(cmd)?;
             let out = child.stdout.take().unwrap();
             let err = child.stderr.take().unwrap();
+            // close stdin to prevent deadlock
+            drop(child.stdin.take().unwrap());
             read2(out, err, &mut |is_out, data, eof| {
                 let pos = if is_out {
                     &mut stdout_pos