Woocommerce – get product in category with specific attribute

Finally I get the solution, this is how you need to query on an attribute:

$query_args['tax_query'] = array(
    array(
        'key'     => 'pa_brand',
        'field'   => 'slug',
        'terms'   => 'nike'
    )
);

Note that in the key you need to add pa_ to your attribute name and you need to use terms to specify the value of your attribute.