allow exec command after --workspace-folder flag

It seems that the 'exec' command is only allowed as the first parameter.

```
devcontainer --workspace-folder . exec ls
OCI runtime exec failed: exec failed: unable to start container process: exec: "exec": executable file not found in $PATH: unknown
```

Like `up` etc., it expects options followed by `exec` to be allowed.

With this change, one can create an alias as `alias devc="devcontainer --workspace-folder PATH"`.