goaction.Setenv is using a deprecated API

[posener]

The testworkflow.yml fails on the following error:

Error: Unable to process command '::set-env name=set::set' successfully.
Error: The `set-env` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/
Error: Unable to process command '::set-env name=export::export' successfully.
Error: The `set-env` command is disabled. Please upgrade to using Environment Files or opt into unsecure command execution by setting the `ACTIONS_ALLOW_UNSECURE_COMMANDS` environment variable to `true`. For more information see: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

Instructions in https://docs.github.com/en/actions/reference/workflow-commands-for-github-actions#environment-files

Env should be appended to a file located under $GITHUB_ENV path as <key>=<value>\n.