Add source link for DEFAULT_GO_PACKAGES taskfile variable

These variables come from a reusable asset that is maintained in a centralized repository. Documenting its source will
facilitate pulling in changes from development in the upstream repository, and also pushing fixes or enhancements that
are made to it via this project.

Author: per1234

Taskfile.yml

@@ -5,6 +5,7 @@ includes:
   dist: ./DistTasks.yml
 
 vars:
+  # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/go-task/Taskfile.yml
   # Path of the project's primary Go module:
   DEFAULT_GO_MODULE_PATH: ./
   DEFAULT_GO_PACKAGES: