Tasks: Pick folder for user tasks in multi-root-workspaces

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> User-Tasks in Multi-Root-Workspaces should ask in which workspace Folder they should be executed. Currently every User-Task will only be executed in the first workspaceFolder.