os/exec: fix variable shadow, don't leak goroutine

Goroutine leak checking is still too tedious, so untested.

See #6705 which is my fault for forgetting to mail out.

Change-Id: I899fb311c9d4229ff1dbd3f54fe307805e17efee
Reviewed-on: https://go-review.googlesource.com/22581
Reviewed-by: Ahmed W. <oneofone@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/exec/exec.go

@@ -412,7 +412,7 @@ func (c *Cmd) WaitContext(ctx context.Context) error {
 
 	var waitDone chan struct{}
 	if ctx != nil {
-		waitDone := make(chan struct{})
+		waitDone = make(chan struct{})
 		go func() {
 			select {
 			case <-ctx.Done():