git.origin -> git.github_pr_destination fails when specifying a SHA-1 `ref` on the origin

I'm attempting to write a macro for a workflow which will take a public Git SoT (`git.origin` is necessary, since this can be sources other than `github` or `gerrit`). I'd like to submit this import as a PR to a private github destination (via `git.github_pr_destination`).

The workflow that gets generated is pretty simple:

```
core.workflow(
name = "some_foo_name",
mode = "SQUASH",
origin = git.origin(
url = url,
ref = "someCommitHash",
),
destination = git.github_pr_destination(
url = private_url,
destination_ref = "master",
pr_branch = "import/some_foo_name/${CONTEXT_REFERENCE}",
),

```

However, when running this workflow, I get a error stating that I need to specify the `pr_branch` for this:

```
ERROR: git.github_pr_destination is incompatible with the current origin. Origin has to be able to provide the contextReference or use '--github-destination-pr-branch' flag
```

Am I going about this incorrectly? What is a more idiomatic way to do this?