Workflow body parameters?

I want my workflow to be easily configurable. e.g. someone can take it, then change a branch name or repo name in one place, and it applies everywhere.

At the moment I'm doing that with env vars:
> Build a report for ${PRODUCT}

Then there is also a note in the workflow body, e.g.
> `${PRODUCT}` matches to `Contoso`.

Is there a better way to do this? Ideally the placeholder in the agent body would be replaced before it is send to the LLM, e.g. `Build a report for Contoso` and not `Build a report for ${PRODUCT}`.

It's not clear to be whether env vars do that today or not. I want them replaced before the agent is called so replacement is deterministic.