Restore Cache.flutterRoot after overriding it in a test (#93555)

Fixes https://github.com/flutter/flutter/issues/93553

packages/flutter_tools/test/general.shard/cache_test.dart

@@ -929,7 +929,9 @@ void main() {
     });
 
     testUsingContext('AndroidMavenArtifacts can invoke Gradle resolve dependencies if Android SDK is present', () async {
+      final String oldRoot = Cache.flutterRoot;
       Cache.flutterRoot = '';
+      try {
         final AndroidMavenArtifacts mavenArtifacts = AndroidMavenArtifacts(cache, platform: FakePlatform());
         expect(await mavenArtifacts.isUpToDate(memoryFileSystem), isFalse);
 
@@ -941,6 +943,9 @@ void main() {
 
         expect(await mavenArtifacts.isUpToDate(memoryFileSystem), isFalse);
         expect(fakeAndroidSdk.reinitialized, true);
+      } finally {
+        Cache.flutterRoot = oldRoot;
+      }
     }, overrides: <Type, Generator>{
       Cache: () => cache,
       FileSystem: () => memoryFileSystem,