Fix cwd flag for ctr tasks exec

It seems like the cwd flag isn't used anywhere for ctr tasks exec. This change
just sets the cwd field on the spec for the execed process if a new one was
asked for, otherwise it will continue using whatever was on the containers spec.

Signed-off-by: Daniel Canter <dcanter@microsoft.com>

cmd/ctr/commands/tasks/exec.go

@@ -104,6 +104,10 @@ var execCommand = cli.Command{
 		pspec.Terminal = tty
 		pspec.Args = args
 
+		if cwd := context.String("cwd"); cwd != "" {
+			pspec.Cwd = cwd
+		}
+
 		task, err := container.Task(ctx, nil)
 		if err != nil {
 			return err