Break out macOS test into a separate job

Turns out the underlying setup-beam workflow doesn't support macOS [1].

This change breaks out that OS into its own job whereby we install
Erlang and (some version of) Elixir using Homebrew. I derived this
solution from [2].

🤞🏻 Let's see if it works!

[1] erlef/setup-beam#54
[2] https://github.com/rusterlium/rustler/blob/master/.github/workflows/main.yml

.github/workflows/ci.yml

@@ -7,25 +7,36 @@ on:
     branches: ["main"]
   workflow_dispatch:
 
+env:
+  MIX_ENV: test
+
 jobs:
-  test:
-    name: Elixir ${{ matrix.pair.elixir }} OTP ${{ matrix.pair.otp }} (${{ matrix.os }})
-    runs-on: ${{ matrix.os }}
-    env:
-      MIX_ENV: test
+  test_macos:
+    name: OTP ${{ matrix.otp }} (macOS)
+    runs-on: macos_latest
+    strategy:
+      fail-fast: false
+      matrix:
+        otp: ["erlang@24", "erlang"]
+      steps:
+        - uses: actions/checkout@v4
+        - name: Install Erlang and Elixir with Homebrew
+          run: |
+            brew install ${{ matrix.otp }}
+            mix local.hex --force
+        - run: mix format --check-formatted
+        - run: mix test
+  test_ubuntu:
+    name: Elixir ${{ matrix.pair.elixir }} OTP ${{ matrix.pair.otp }} (Ubuntu)
+    runs-on: ubuntu_latest
     strategy:
       fail-fast: false
       matrix:
-        os:
-          - ubuntu-latest
-          - macos-latest
-        include:
-          - pair:
-              elixir: "1.13"
-              otp: "24.3.4.10"
-          - pair:
-              elixir: "1.17"
-              otp: "27.0.1"
+        pair:
+          - elixir: "1.13"
+            otp: "24.3.4.10"
+          - elixir: "1.17"
+            otp: "27.0.1"
     steps:
       - uses: actions/checkout@v4
       - uses: CargoSense/setup-elixir-project@v1