@kimdv looks great so far! Not sure if you want to add any other feature to the step, right now it will install swiftlint and run it in the current directory - maybe an input for where swiftlint lint should be run would be useful? Just an idea 
Dev note: as described at GitHub - kimdv/bitrise-step-swiftlint you can now use this step both on your Mac and on bitrise.io, without even sharing it. On your Mac you can use the path::/path/to/step form, and the git:: one works everywhere even on bitrise.io - example:
workflows:
primary:
steps:
- activate-ssh-key@3.1.1:
run_if: '{{getenv "SSH_RSA_PRIVATE_KEY" | ne ""}}'
- git-clone@3.5.1: {}
- git::https://github.com/kimdv/bitrise-step-swiftlint.git: {}
P.S.: you can run the workflow editor on your Mac too, with: bitrise :workflow-editor (Workflow Editor "v2" (Open Source & Offline Workflow Editor))