ci: add workflow tests and fix concurrency group

[sumitsaini03]

• tests/test_build_workflow.py: matrix/script consistency, dup job names, concurrency group, structural sanity

• requirements-dev.txt: PyYAML for the workflow tests

• build.yml: replace 'pull_request.number || github.ref' with workflow+event_name+ref so non-PR triggers don't share a slot under cancel-in-progress: true

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[baranovmv]

Thanks for contribution. Unfortunately I'm closing the PR as it lacks

• motivation for the changes -- I mean, some of it might be useful, but it'd be better to bring an explicit explanation what lead you to introducing these changes, an issue would be perfect, but not required;
• and coherency -- keep PR small and related to a single purpose, fix or feature