Fix Style/FetchEnvVar rubocop violations

The latest version of rubocop as of this commit (v1.28.1) was reporting
these three violations:
```
Rakefile:15:19: C: [Correctable] Style/FetchEnvVar: Use ENV.fetch('EXAMPLE_APP') or ENV.fetch('EXAMPLE_APP', nil) instead of ENV['EXAMPLE_APP'].
    example_app = ENV['EXAMPLE_APP'] || 'demo_app'
                  ^^^^^^^^^^^^^^^^^^
Rakefile:58:21: C: [Correctable] Style/FetchEnvVar: Use ENV.fetch('INTEGRATION_BUNDLE_ARGS') or ENV.fetch('INTEGRATION_BUNDLE_ARGS', nil) instead of ENV['INTEGRATION_BUNDLE_ARGS'].
      bundle_args = ENV['INTEGRATION_BUNDLE_ARGS']
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
test/external_asset_pipeline/dev_server_test.rb:16:21: C: [Correctable] Style/FetchEnvVar: Use ENV.fetch('DEV_SERVER_HOST') or ENV.fetch('DEV_SERVER_HOST', nil) instead of ENV['DEV_SERVER_HOST'].
      config.host = ENV['DEV_SERVER_HOST'] || 'localhost'
                    ^^^^^^^^^^^^^^^^^^^^^^
test/external_asset_pipeline/dev_server_test.rb:32:21: C: [Correctable] Style/FetchEnvVar: Use ENV.fetch('DEV_SERVER_HOST') or ENV.fetch('DEV_SERVER_HOST', nil) instead of ENV['DEV_SERVER_HOST'].
      config.host = ENV['DEV_SERVER_HOST'] || 'localhost'
                    ^^^^^^^^^^^^^^^^^^^^^^
test/external_asset_pipeline/dev_server_test.rb:50:21: C: [Correctable] Style/FetchEnvVar: Use ENV.fetch('DEV_SERVER_HOST') or ENV.fetch('DEV_SERVER_HOST', nil) instead of ENV['DEV_SERVER_HOST'].
      config.host = ENV['DEV_SERVER_HOST'] || 'localhost'
                    ^^^^^^^^^^^^^^^^^^^^^^
test/integration/external_asset_pipeline_integration_test.rb:29:23: C: [Correctable] Style/FetchEnvVar: Use ENV.fetch('DEV_SERVER_HOST') or ENV.fetch('DEV_SERVER_HOST', nil) instead of ENV['DEV_SERVER_HOST'].
    dev_server_host = ENV['DEV_SERVER_HOST'] || 'localhost'
                      ^^^^^^^^^^^^^^^^^^^^^^
```

Author: rmacklin

Rakefile

@@ -12,7 +12,7 @@ class ExampleAppCommand
 
   def initialize(command)
     @command = command
-    example_app = ENV['EXAMPLE_APP'] || 'demo_app'
+    example_app = ENV.fetch('EXAMPLE_APP', 'demo_app')
     @example_app_path = File.expand_path("./examples/#{example_app}", __dir__)
     @gemfile_path = File.join(@example_app_path, 'Gemfile')
   end
@@ -55,7 +55,7 @@ namespace :test do
   namespace :integration do
     desc 'Resolve and install dependencies and build assets in example app'
     task :prepare do
-      bundle_args = ENV['INTEGRATION_BUNDLE_ARGS']
+      bundle_args = ENV.fetch('INTEGRATION_BUNDLE_ARGS', '')
       ExampleAppCommand.run(
         "bundle check #{bundle_args} || bundle install #{bundle_args}"
       )

test/external_asset_pipeline/dev_server_test.rb

@@ -13,7 +13,7 @@ module ExternalAssetPipeline
   class DevServerTest < Minitest::Test
     def test_get
       config = Configuration::DevServerSettings.new
-      config.host = ENV['DEV_SERVER_HOST'] || 'localhost'
+      config.host = ENV.fetch('DEV_SERVER_HOST', 'localhost')
       config.port = 9555
 
       dev_server = DevServer.new(config)
@@ -29,7 +29,7 @@ def test_get
 
     def test_origin
       config = Configuration::DevServerSettings.new
-      config.host = ENV['DEV_SERVER_HOST'] || 'localhost'
+      config.host = ENV.fetch('DEV_SERVER_HOST', 'localhost')
       config.port = 9666
 
       dev_server = DevServer.new(config)
@@ -47,7 +47,7 @@ def test_origin
 
     def test_running
       config = Configuration::DevServerSettings.new
-      config.host = ENV['DEV_SERVER_HOST'] || 'localhost'
+      config.host = ENV.fetch('DEV_SERVER_HOST', 'localhost')
       config.port = 9777
 
       dev_server = DevServer.new(config)

test/integration/external_asset_pipeline_integration_test.rb

@@ -26,7 +26,7 @@ class ExternalAssetPipelineIntegrationTest < ActionDispatch::IntegrationTest
 
     Rails.application.config.external_asset_pipeline.cache_manifest = false
     dev_server_thread = create_server_thread(9000)
-    dev_server_host = ENV['DEV_SERVER_HOST'] || 'localhost'
+    dev_server_host = ENV.fetch('DEV_SERVER_HOST', 'localhost')
     wait_for_server(dev_server_host, 9000)
 
     get root_url