Build RNTester with Fabric when env variable 'USE_FABRIC' is set

packages/rn-tester/Podfile

@@ -13,16 +13,25 @@ def pods()
 
   # Enable TurboModule
   prefix_path = "../.."
-  use_react_native!(path:prefix_path)
+  fabric_enabled = false
+
+  # To use fabric: set the environment variable `USE_FABRIC` to 1, like below
+  #   $ USE_FABRIC=1 bundle exec pod install
+  # or
+  #   $ export USE_FABRIC=1
+  #   $ bundle exec pod install
+  if ENV['USE_FABRIC'] == '1'
+    puts "Building RNTester with Fabric enabled."
+    fabric_enabled = true
+  end
+
+  use_react_native!(path:prefix_path, fabric_enabled: fabric_enabled)
   pod 'ReactCommon/turbomodule/samples', :path => "#{prefix_path}/ReactCommon"
 
   # Additional Pods which aren't included in the default Podfile
   pod 'React-RCTPushNotification', :path => "#{prefix_path}/Libraries/PushNotificationIOS"
   pod 'Yoga',:path => "#{prefix_path}/ReactCommon/yoga", :modular_headers => true
   # Additional Pods which are classed as unstable
-  #
-  # To use fabric: add `fabric_enabled` option to the use_react_native method above, like below
-  # use_react_native!(path: "..", fabric_enabled: true)
 end
 
 target 'RNTester' do