Predefined `Build.SourceBranch` variable not working

Hi,

We're trying to migrate from `pr` triggers in the `azure-pipeline.yml` to a GitHub Action trigger, using the following action:
```
...
azure_pipeline:
needs: [previous_action]
runs-on: ubuntu-latest
steps:
- name: Azure Pipelines Action
uses: Azure/pipelines@v1
with:
azure-devops-project-url: https://dev.azure.com/organization/project
azure-pipeline-name: cogz-ci
azure-devops-token: ${{ secrets.AZURE_DEVOPS_TOKEN }}
```
The trigger works great, but some of the build information is missing.

For example, our build name in the `azure-pipeline.yml` file is defined as follows:
`name: cogz-ci-$(branchName)-$(Date:yyyyMMdd)$(Rev:.rr)`
`branchName` is defined in a global template as:
```
${{ if startsWith(variables['Build.SourceBranch'], 'refs/pull/') }}:
branchName: $[ replace(replace(replace(variables['system.pullRequest.sourceBranch'], 'refs/heads/', ''), '\', '-'), '/', '-') ]
${{ if startsWith(variables['Build.SourceBranch'], 'refs/heads/') }}:
branchName: $[ replace(replace(replace(variables['build.sourceBranch'], 'refs/heads/', ''), '\', '-'), '/', '-') ]
```

When running with the azure pipelines PR trigger, we get:

But when running with the GitHub Action trigger, we get:
(the build also fails because we're using `branchName` in a lot of places)

Are we missing something? Should this be passed as a variable when using the GitHub Action trigger?

Thanks!