Fix System Tests in CI environment

Resolve breaking changes introduced in `@hotwired/turbo` that were later
fixed in [@hotwired/turbo#4f334da][] (and further resolved in
[@hotwired/turbo#7a7c6e2][]).

CI is failing because those changes are not available as part of a beta
release. Despite the fact that these changes wouldn't be necessary if
that release were available, this resolves the broken build pipeline
more immediately.

[@hotwired/turbo#4f334da]: hotwired/turbo@4f334da
[@hotwired/turbo#7a7c6e2]: hotwired/turbo@7a7c6e2

test/dummy/app/javascript/application.js

@@ -1,2 +1,4 @@
 // Configure your import map in config/importmap.rb. Read more: https://github.com/rails/importmap-rails
-import "@hotwired/turbo-rails"
+import { Turbo } from "@hotwired/turbo-rails"
+
+window.StreamActions = Turbo.StreamActions

test/system/broadcasts_test.rb

@@ -134,7 +134,7 @@ def assert_broadcasts_text(text, to:, &block)
 
   def assert_forwards_turbo_stream_tag_attribute(attr_key:, attr_value:, to:, &block)
     execute_script(<<~SCRIPT)
-      Turbo.StreamActions.test = function () {
+      StreamActions.test = function () {
         const attribute = this.getAttribute('#{attr_key}')
 
         document.getElementById('#{to}').innerHTML = attribute

yarn.lock

@@ -206,9 +206,9 @@ hasown@^2.0.0:
   dependencies:
     function-bind "^1.1.2"
 
-"idiomorph@git+https://github.com/basecamp/idiomorph.git#rollout-build":
+"idiomorph@https://github.com/basecamp/idiomorph#rollout-build":
   version "0.0.8"
-  resolved "git+https://github.com/basecamp/idiomorph.git#e906820368e4c9c52489a3336b8c3826b1bf6de5"
+  resolved "https://github.com/basecamp/idiomorph#e906820368e4c9c52489a3336b8c3826b1bf6de5"
 
 is-core-module@^2.13.0:
   version "2.13.1"