rule export: set `AUGUR_RECURSION_LIMIT` to 20000

## Description of proposed changes The h5n1-cattle-outbreak/mp build hit a RecursionError during the validation of the produced Auspice JSON. Works around <https://github.com/nextstrain/avian-flu/issues/151>, but maybe there's a better way to resolve this. ## Checklist <!-- Make sure checks are successful at the bottom of the PR. If applicable, add: - any changes to existing tests - any additional manual testing to confirm changes Please add a note if you need help with adding tests. --> - [x] Checks pass <!-- 🙌 Thank you for contributing to Nextstrain! ✨ -->