Add CI matrix for macOS builds (#43)

* Add FFmpeg version matrix to macOS CI job

* Add macOS PKG_CONFIG_PATH

* Change schedule to run CI builds weekly

* Fix package installation dir

Homebrew apparently always symlinks to /opt/homebrew, so this
one is reliable

.github/workflows/build.yml

@@ -3,7 +3,7 @@ on:
   push:
   pull_request:
   schedule:
-    - cron: "0 0 * * *" # Daily
+    - cron: "20 7 * * 4" # Weekly on thursday 
 
 env:
   CARGO_INCREMENTAL: 0
@@ -65,12 +65,20 @@ jobs:
         run: cargo test $CARGO_FEATURES
 
   build-test-lint-macos:
-    name: macOS - FFmpeg latest - build, test and lint
+    name: macOS - FFmpeg ${{ matrix.ffmpeg_version }} - build, test and lint
     runs-on: macos-latest
+    strategy:
+      matrix:
+        ffmpeg_version: ["4", "5", "6"]
+      fail-fast: false
+
+    env:
+      PKG_CONFIG_PATH: /opt/homebrew/opt/ffmpeg@${{ matrix.ffmpeg_version }}/lib/pkgconfig
+
     steps:
       - uses: actions/checkout@v4
       - name: Install dependencies
-        run: brew install ffmpeg pkg-config
+        run: brew install ffmpeg@${{ matrix.ffmpeg_version }} pkg-config
       - name: Install Rust stable with clippy and rustfmt
         uses: dtolnay/rust-toolchain@stable
         with: