Update Flutter plugin URL template for downloads (#189)

Author: heliocodacy

plugins/runtimes/flutter/plugin.yaml

@@ -2,7 +2,7 @@ name: flutter
 description: Dart Flutterruntime
 default_version: "3.7.2"
 download:
-  url_template: "https://storage.googleapis.com/flutter_infra_release/releases/stable/{{.OS}}/flutter_{{.OS}}_{{.Arch}}_{{.Version}}-stable.zip"
+  url_template: "https://storage.googleapis.com/flutter_infra_release/releases/stable/{{.OS}}/flutter_{{.OS}}_{{.Arch}}_{{.Version}}-stable.{{.Extension}}"
   file_name_template: "flutter"
   extension:
     default: "zip"