A GitHub Action to download a binary release of Dafny and add it to the system $PATH.

- name: "Install Dafny"
  uses: dafny-lang/setup-dafny-action@v1

If you need to use a specific version:

- name: "Install Dafny"
  uses: dafny-lang/setup-dafny-action@v1
  with:
    dafny-version: "2.3.0"

This action transparently works on macOS by detecting the running OS. You can just set runs-on to a macOS virtual environment like macos-latest.