Fix an ordering dependency in the flutter_tools upgrade test (#129131)

Cache.flutterRoot is set within testUsingContext and will be
uninitialized the first time test suite setup is invoked.

packages/flutter_tools/test/commands.shard/permeable/upgrade_test.dart

@@ -38,7 +38,7 @@ void main() {
     setUp(() {
       fakeCommandRunner = FakeUpgradeCommandRunner();
       realCommandRunner = UpgradeCommandRunner()
-          ..workingDirectory = Cache.flutterRoot;
+          ..workingDirectory = getFlutterRoot();
       processManager = FakeProcessManager.empty();
       fakeCommandRunner.willHaveUncommittedChanges = false;
       fakePlatform = FakePlatform()..environment = Map<String, String>.unmodifiable(<String, String>{