chore: add `mix_audit` #36

Job	Run time
ash-ci / mix deps.get	20s
ash-ci / MIX_ENV=test mix.compile	1m 48s
ash-ci / audit	1m 51s
ash-ci / MIX_ENV=dev mix.compile	1m 40s
ash-ci / mix docs	35s
ash-ci / mix sobelow	41s
ash-ci / mix dialyzer	3m 24s
ash-ci / mix format --check-formatted	32s
ash-ci / mix credo --strict	3s
ash-ci / mix spark.cheat_sheets --check	38s
ash-ci / mix spark.formatter --check	5s
ash-ci / mix test	1m 7s
ash-ci / mix deps.unlock --check-unused	30s
ash-ci / Deploy docs to GitHub pages.	8s
ash-ci / Release	0s
	13m 22s

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