Fix issue when deploying linux client with no output parameter

When deploying the linux client, if neither the `output` nor the `outputdir` parameter is specified, the `.deb` file is written in a temporary directory and deleted at the end of the execution. The execution finishes normally without any error message.
This issue appeared with the new `outputdir` parameter in 71edc3e4.

This pull request should solve this: if neither `output` nor `outputdir` is specified, `None` is passed as the output parameter to `deployer.MakeDeployableBinary()`. Thus, the default location from the configuration is used (`ClientBuilder.output_path` in [build.py](https://github.com/google/grr/blob/master/lib/build.py#L580-L582)). This was the default behaviour prior to 71edc3e4.

/cc @destijl