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~~ use ASDF to install
the matrix'ed versions of Elixir and Erlang. I (originally) 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

Squashed WIP commits with the following commit messages:

- 0a42b83 YAML 🥴
- 5b9a38a Fix runs-on values
- 95f2fd8 Try using asdf reusable workflow to install Elixir and Erlang
- 1785e90 Update macOS job name
- 3277cef Install Hex
- 5d1d30d Install dependencies in macOS jobs

.github/workflows/ci.yml

@@ -7,25 +7,46 @@ 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: Elixir ${{ matrix.pair.elixir }} OTP ${{ matrix.pair.otp }} (macOS)
+    runs-on: macos-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        pair:
+          - elixir: "1.13"
+            otp: "24.3.4.10"
+          - elixir: "1.17"
+            otp: "27.0.1"
+    steps:
+      - uses: actions/checkout@v4
+      - name: Generate .tool-versions file
+        run: |
+          echo "elixir ${{ matrix.pair.elixir }}" >> .tool-versions
+          echo "erlang ${{ matrix.pair.otp }}" >> .tool-versions
+          cat .tool-versions
+      - uses: asdf-vm/actions/install@v3
+      - name: Install Hex package manager
+        run: mix local.hex --force
+      - name: Install dependencies
+        run: mix deps.get
+      - 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