syscall: use correct file descriptor in dup2 fallback path

This fixes a mistake in CL 220422. This changes code that is only
executed on Linux kernel versions earlier than 2.6.27.

Change-Id: I01280184f4d7b75e06387c38f1891e8f0a81f793
Reviewed-on: https://go-review.googlesource.com/c/go/+/244630
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

src/syscall/exec_linux.go

@@ -465,7 +465,7 @@ func forkAndExecInChild1(argv0 *byte, argv, envv []*byte, chroot, dir *byte, att
 			}
 			_, _, err1 = RawSyscall(SYS_DUP3, uintptr(fd[i]), uintptr(nextfd), O_CLOEXEC)
 			if _SYS_dup != SYS_DUP3 && err1 == ENOSYS {
-				_, _, err1 = RawSyscall(_SYS_dup, uintptr(pipe), uintptr(nextfd), 0)
+				_, _, err1 = RawSyscall(_SYS_dup, uintptr(fd[i]), uintptr(nextfd), 0)
 				if err1 != 0 {
 					goto childerror
 				}