Don't attempt to warm the Capybara cache outside of CI

We initially started doing this because the first feature test to use
the JavaScript driver would often time out due to asset compilation on
CI. We should still do it in CI since the cost is (relatively) low, but
doing so for every local run is both tiresome and generally unnecessary,
as developer machines will be faster than most shared CI runners, and
the cost of retrying a test after a timeout is much, much lower.

spec/support/test_env.rb

@@ -206,6 +206,7 @@ module TestEnv
   # Otherwise they'd be created by the first test, often timing out and
   # causing a transient test failure
   def eager_load_driver_server
+    return unless ENV['CI']
     return unless defined?(Capybara)
 
     puts "Starting the Capybara driver server..."