Add flake8 to synced workflows

[surchs]

All our python repos (should) have the same linting rules and also run flake8. So they should be getting this workflow from the workflows repo.

Related:

• https://github.com/neurobagel/planning/issues/143
• https://github.com/neurobagel/project/issues/74
• https://github.com/neurobagel/workflows/issues/90

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