Document go:test task's environment variable parameters

The behavior of the `go:test` task can be configured via environment variables. It is important to document these
parameter variables in order to make their existence known to contributors who would find them useful.

Previously, the parameter environment variables of this task were undocumented.

Author: per1234

workflow-templates/assets/test-go-task/Taskfile.yml

@@ -9,7 +9,11 @@ vars:
 tasks:
   # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/test-go-task/Taskfile.yml
   go:test:
-    desc: Run unit tests
+    desc: |
+      Run unit tests.
+      Environment variable parameters:
+      - GO_MODULE_PATH: Path of the Go module root (default: {{.DEFAULT_GO_MODULE_PATH}}).
+      - GO_PACKAGES: List of Go packages to test (default: all packages of the module).
     dir: "{{default .DEFAULT_GO_MODULE_PATH .GO_MODULE_PATH}}"
     cmds:
       - |