Fix fallback input for release action

Apparently the way submodules repos are configured leads to the branches not being mirrored locally (?) As such, the release job's fallback of checking out `'master'` fails whether triggered[^1] or scheduled[^2].

[^1]: https://github.com/ua-parser/uap-python/actions/runs/13090871627
[^2]: https://github.com/ua-parser/uap-python/actions/runs/13092233962

Author: masklinn

.github/workflows/release-builtins.yml

@@ -28,7 +28,7 @@ jobs:
         persist-credentials: false
     - name: update core
       env:
-        TAG: ${{ inputs.tag || 'master '}}
+        TAG: ${{ inputs.tag || 'origin/master '}}
       # needs to detach because we can update to a tag
       run: git -C uap-core switch --detach "$TAG"
     - name: Set up Python