Reduce OA clones in GitHub Actions builds

[MaxGhenis]

Summary

• parameterize the OA clone count in dataset creation via PE_UK_DATA_OA_CLONES
• set GitHub Actions dataset builds to use a single OA assignment per household
• add a changelog fragment for the publication fix

Why

Recent Push publication runs on main have been dying inside make data during constituency calibration, before Upload data starts. The regression lines up with the OA calibration pipeline landing and the default build path cloning each household 10x before calibration.

This keeps the OA assignment path available while making the hosted-runner build fit again.

Validation

• python -m compileall policyengine_uk_data/datasets/create_datasets.py
• PYTHONPATH=/Users/maxghenis/policyengine-uk uv run --python 3.13 --with ".[dev]" pytest policyengine_uk_data/tests/test_clone_and_assign.py -q
• PYTHONPATH=/Users/maxghenis/policyengine-uk uv run --python 3.13 --with ".[dev]" pytest policyengine_uk_data/tests/test_build_environment.py -q