Issue Title

The task

**Steps to reproduce:** 1. Create a Taskfile with: ```yaml vars: FRAMEWORK_DEP_FILE_LIST: ["a", "b", "c"] FRAMEWORK_DEP_FILES: ref: .FRAMEWORK_DEP_FILE_LIST | join "," ``` 2. Run `task` or any task that resolves this variable. 3. Observe the error: ``` template: resolver:1: function "join" not defined ```