Use consistent branch to avoid duplicate automation PRs

We should use a single branch for automation PRs so that we don't end up creating multiple PRs and instead just add commits to any open PRs.

https://github.com/Azure/azure-sdk-tools/blob/main/eng/common/pipelines/templates/archetype-typespec-emitter.yml#L141-L147

Start in eng/common/pipelines/templates/archetype-typespec-emitter.yml at lines 141-147 and trace how automation PR branches are selected. Confirm the current behavior that can create multiple PRs, then verify that one consistent branch is reused so new commits go to an open PR instead of creating duplicates.

Written by the indexing model from the issue text.