Fix upmerge docs to edge workflow

[sk593]

What content needs to be created or modified?

The upmerge docs workflow needs to be fixed. It fails to commit changes from the previous branch

https://github.com/radius-project/docs/actions/workflows/upmerge.yaml

Describe the solution you'd like

Where should the new material be placed?

The associated pull request from the Radius code repo, if applicable

Additional context

AB#12891

[azure-boards]

✅ Successfully linked to Azure Boards work item(s):

• GitHub Issue 12891

[sylvainsf]

@sk593 Could you provide an example regression?

[brooke-hamilton]

/assign @brooke-hamilton

[sk593]

@sk593 Could you provide an example regression?

Looks like the logs expired but the failure was similar to this:

Run export SOURCE_BRANCH=$(basename refs/heads/v0.35)
Upmerging docs from v0.35 to edge
From https://github.com/radius-project/docs
 * branch            v0.35      -> FETCH_HEAD
<change info for updated code>
On branch upmerge/2024-07-18-1fd3
nothing to commit, working tree clean
Error: Process completed with exit code 1.