rune/libenclave: Close exec fifo fd twice

Unlike what is done in the process of initialization of container
entrypoint, the exec fifo fd is not closed without closing it
explicitly, resulting in rune start cannot be terminated.
Signed-off-by: Jia Zhang <zhang.jia@linux.alibaba.com>

rune/libenclave/runelet.go

@@ -226,6 +226,7 @@ func finalizeInitialization(fifoFd int) error {
 	// since been resolved.
 	// https://github.com/torvalds/linux/blob/v4.9/fs/exec.c#L1290-L1318
 	unix.Close(fifoFd)
+	unix.Close(fd)
 	return nil
 }