Disable warming of the asset cache in Spinach tests under CI

I suspect some combination of Knapsack tests cause no regular Rack tests
to be loaded (i.e. all JavaScript tests), which leads to the error:

ArgumentError: rack-test requires a rack application, but none was given

In CI, we precompile all the assets so there is no need to warm the
asset cache in any case.

Closes #23613

features/support/capybara.rb

@@ -20,5 +20,5 @@ unless ENV['CI'] || ENV['CI_SERVER']
 end
 
 Spinach.hooks.before_run do
-  TestEnv.warm_asset_cache
+  TestEnv.warm_asset_cache unless ENV['CI'] || ENV['CI_SERVER']
 end