Dummy App generator: Use db var instead of ENV var

A couple of lines before this change, we already normalize `ENV['db']`.
We lose that work by relying on the raw ENV var here.

core/lib/generators/spree/dummy/templates/rails/database.yml

@@ -19,7 +19,7 @@
 <% db_username = ENV['DB_USERNAME'] %>
 <% db_password = ENV['DB_PASSWORD'] %>
 
-<% case ENV['DB']
+<% case db
   when 'mysql' %>
 development:
   adapter: mysql2