Ability to update a workspace variable.

Our team is using the tfc-workflows-github create-run to [overwrite a workspace variable](https://github.com/hashicorp/tfc-workflows-github/blob/main/actions/create-run/README.md#tf_var_-environment-variables) and to kick a run. We need a way/workflow to update the workspace variable via the github action workflow.

The terraform config files are in a different repository than from where the github actions are triggered. We are using a TF_VAR_* env var at the moment, but that only overwrites the variable for that run, but our requirement is that it is persisted in the workspace variables as well.