test_runner_formula: fix caching

Library/Homebrew/test_runner_formula.rb

@@ -91,7 +91,7 @@ class TestRunnerFormula
   def dependents(platform:, arch:, macos_version:)
     cache_key = :"#{platform}_#{arch}_#{macos_version}"
 
-    @dependent_hash.fetch(cache_key) do
+    @dependent_hash[cache_key] ||= begin
       all = eval_all || Homebrew::EnvConfig.eval_all?
       formula_selector, eval_all_env = if all
         [:all, "1"]
@@ -112,5 +112,7 @@ class TestRunnerFormula
         Homebrew::SimulateSystem.clear
       end
     end
+
+    @dependent_hash.fetch(cache_key)
   end
 end