Is it possible to use environment variables inside copy.bara.sky file?

Let us say that we have some information (like the commit header) in a environment variable of the system in which copybara runs (say `COMMIT_MESSAGE_ENV`)

Is it possible to refer to that environment variable in the metadata transformations.

```
core.workflow(
name = "default",
origin = git.github_pr_origin(
....
),

destination = git.github_pr_destination(
....
),

....

transformations = [
metadata.add_header("COMMIT_MESSAGE_ENV should be present here"),
],
)
```

Currently, we are able to refer only the implicit labels like `GITHUB_PR_NUMBER`. But I wonder if there is a way to reference just the environment variables inside the copy.bara.sky file.