Run tests on Ubuntu and cancel earlier PR runs

[anth-volk]

Fixes #545

Summary

• Run the pull-request full test matrix on ubuntu-latest.
• Run the post-merge full test matrix on ubuntu-latest.
• Keep the Python 3.11 through 3.14 matrices, installation steps, and test commands unchanged.
• Cancel an earlier pull-request workflow run when the same workflow receives a newer commit on the same PR.
• Keep different PRs, different workflows, manual runs, and post-merge runs independent.

Cancellation scope

The code, documentation, and draft-paper workflows use the following group:

group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.run_id }}
cancel-in-progress: ${{ github.event_name == 'pull_request' }}

The workflow name prevents one workflow from canceling another workflow on the same PR. The PR number prevents one PR from affecting another. Manual and push events use the unique run ID and do not cancel one another.

The post-merge workflow has no new concurrency configuration.

Context

Each full test matrix creates four jobs. GitHub limits the PolicyEngine Team organization to five concurrent macOS jobs, so the previous configuration could delay validation and package publication even when Ubuntu capacity remained available. Git history shows that PolicyEngine.py adopted macOS to match PolicyEngine-UK, without documenting a PolicyEngine.py-specific macOS requirement.

Testing

• make format
• make lint
• actionlint .github/workflows/pr_code_changes.yaml .github/workflows/pr_docs_changes.yaml .github/workflows/draft-pdf.yml
• git diff --check
• uv run make test — 1,188 passed and 9 skipped

actionlint .github/workflows/push.yaml parses the runner change but reports the existing actions/configure-pages@v3 version on main as unsupported.