Setup initial GitHub CI to build on macos-12.

GitHub provides this as x86_64 Mac.

Author: jwcullen

.github/workflows/ci.yml

@@ -19,4 +19,12 @@ jobs:
         uses: ./.github/actions/libiamf-builder
         with:
           build-options: -DCODEC_CAP=off
-
+  macos-amd64:
+    runs-on: macos-12
+    steps:
+      - name: Checkout repo
+        uses: actions/checkout@v4
+      - name: Build
+        uses: ./.github/actions/libiamf-builder
+        with:
+          build-options: -DCODEC_CAP=off