ask the routes objects for its Rack env key

this centralizes the logic for determining the script name key and drops
object allocations when calling `engine_script_name` (which is called on
each `url_for`).

actionpack/lib/action_dispatch/http/request.rb

@@ -114,7 +114,7 @@ module ActionDispatch
     end
 
     def engine_script_name(_routes) # :nodoc:
-      env["ROUTES_#{_routes.object_id}_SCRIPT_NAME"]
+      env[_routes.env_key]
     end
 
     def request_method=(request_method) #:nodoc:

actionpack/lib/action_dispatch/routing/route_set.rb

@@ -308,6 +308,7 @@ module ActionDispatch
       attr_accessor :formatter, :set, :named_routes, :default_scope, :router
       attr_accessor :disable_clear_and_finalize, :resources_path_names
       attr_accessor :default_url_options, :request_class
+      attr_reader :env_key
 
       alias :routes :set
 
@@ -325,6 +326,7 @@ module ActionDispatch
         @prepend                    = []
         @disable_clear_and_finalize = false
         @finalized                  = false
+        @env_key                    = "ROUTES_#{object_id}_SCRIPT_NAME".freeze
 
         @set    = Journey::Routes.new
         @router = Journey::Router.new @set

railties/lib/rails/engine.rb

@@ -513,7 +513,7 @@ module Rails
     def call(env)
       env.merge!(env_config)
       if env['SCRIPT_NAME']
-        env["ROUTES_#{routes.object_id}_SCRIPT_NAME"] = env['SCRIPT_NAME'].dup
+        env[routes.env_key] = env['SCRIPT_NAME'].dup
       end
       app.call(env)
     end