shipit: precompile manifest and disable compilation fallback

This remove the dependency on having a JS runtime to run the benchmark
and is also closer to how Shipit runs in production.

The first iteration is now much faster.

Author: byroot

benchmarks/shipit/config/application.rb

@@ -31,6 +31,9 @@ class Application < Rails::Application
     # Common ones are `templates`, `generators`, or `middleware`, for example.
     config.autoload_lib(ignore: %w[assets tasks])
 
+    config.assets.compile = false
+    config.assets.manifest = "config/manifest.json"
+
     # Configuration for the application, engines, and railties goes here.
     #
     # These settings can be overridden in specific environments using the files