Add source URL comment to "Test Go" workflow

This will make it easier for the maintainers to sync fixes and improvements in either direction between the upstream
"template" workflow and its installation in this repository.

Author: per1234

.github/workflows/test-go-task.yml

@@ -1,3 +1,4 @@
+# Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/test-go-task.md
 name: Test Go
 
 env:

Taskfile.yml

@@ -28,6 +28,7 @@ tasks:
     cmds:
       - go build -v {{.GO_BUILD_FLAGS}}
 
+  # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/test-go-task/Taskfile.yml
   go:test:
     desc: Run unit tests
     cmds: