Correctly output driven_by setting for Development Containers

Currently, `ENV["SELENIUM_HOST"]` is evaluated when generating a file. So
the result was the following.

```
  driven_by :selenium, using: :headless_chrome, screen_size: [ 1400, 1400 ], options: {
    browser: :remote,
    url: "http://:4444"
  }
```

This PR fixes to output the URL setting just as a string. The after
result is the following.

```
  driven_by :selenium, using: :headless_chrome, screen_size: [ 1400, 1400 ], options: {
    browser: :remote,
    url: "http://#{ENV["SELENIUM_HOST"]}:4444"
  }
```

Author: y-yagi

railties/lib/rails/generators/rails/devcontainer/devcontainer_generator.rb

@@ -148,7 +148,7 @@ def local_rails_mount
         end
 
         def system_test_configuration
-          <<~RUBY
+          <<~'RUBY'
               if ENV["CAPYBARA_SERVER_PORT"]
                 served_by host: "rails-app", port: ENV["CAPYBARA_SERVER_PORT"]