gistfile1.txt

    steps:
      - checkout
      - restore_cache:
          keys:
            - project-{{ checksum "mix.exs" }}-{{ checksum "mix.lock" }}
      - run:
          name: deps
          command: MIX_ENV=test mix deps.get
      - run:
          name: compile test
          command: MIX_ENV=test mix compile
      - save_cache:
          key: project-{{ checksum "mix.exs" }}-{{ checksum "mix.lock" }}
          paths:
            - _build
            - deps
      - run:
          name: test
          command: mix test

Heads up!
Caching _build/deps is HIGHLY not recommended as it has lots of consequence. Example: changing mime for plug will need recompile plug deps. Change in umbrella app with Ecto sandbox will need deps to be recompiled etc.

elixir-ecto/ecto#1355 (comment)
elixir-plug/plug#334
elixir-ecto/postgrex#131

There are many issues around caching _build/deps

Ah! That makes sense, thnaks @noma4i ! Will remove that then, didn't know it had such ramifications.