Usage example for the build-to-release-branch action needs to specify `persist-credentials: false`

I recently adapted the example script from teh `build-to-release-branch` action for use in a plugin, and I found that the "checkout" step was causing problems.

The action performs its own actions/checkout internally [here](https://github.com/humanmade/hm-github-actions/blob/main/.github/actions/build-to-release-branch/action.yml#L40-L44), which sets up git auth headers. My workflow's prior actions/checkout call had set the same headers, causing a duplicate Authorization header.

I was able to work around this problem by specifying `persist-credentials: false` in the first checkout:

https://github.com/humanmade/post-link-block/blob/6c5e316a604cf4a15be21f227a96ea436c6b2f64/.github/workflows/build-release-branch.yml#L19-L20

I think this flag would be good to add to the action documentation.