Respect vault/checkout configuration on cart paypal button

We have a configuration for the preferred payment flow, however it is
only recognised in the checkout paypal payment button, and not in the
cart paypal button. This commit adds that recognition.

app/views/spree/shared/_paypal_cart_button.html.erb

@@ -5,7 +5,7 @@
 <script>
   var paypalOptions = {
     buttonStyle: <%== SolidusPaypalBraintree::Gateway.first.preferred_paypal_button_style.to_json %>,
-    flow: 'vault',
+    flow: '<%= SolidusPaypalBraintree::Gateway.first.preferred_paypal_flow %>',
     enableShippingAddress: true,
     useDataCollector: <%= SolidusPaypalBraintree::Gateway.first.preferred_use_data_collector %>,
     environment: '<%= Rails.env.production? ? "production" : "sandbox" %>'