[bug] Fix devbox exec (#169)

## Summary
Fix devbox exec. I think it is a merge failure

## How was it tested?
```
go build -o devbox cmd/devbox/main.go
devbox build
docker run -p 3000:3000 --expose 3000 -ti devbox
```

Author: LucilleH

devbox.go

@@ -150,7 +150,7 @@ func (d *Devbox) Shell() error {
 }
 
 func (d *Devbox) Exec(cmds ...string) error {
-	plan, err := d.Plan()
+	plan, err := d.ShellPlan()
 	if err != nil {
 		return errors.WithStack(err)
 	}