CI: Test Node 14 with macos-13 image

macOS 14 (`macos-latest`, `macos-14`) runners are implicitly
Apple Silicon runners, at least for runners we have access to
on the free tier.

Node 14 isn't built for Apple Silicon.

So, we can test Node 14 on older macOS, like... macOS 13.
That will get us an x64 macOS runner, an arch for which Node 14
should indeed be available.

.github/workflows/ci.yml

@@ -21,9 +21,13 @@ jobs:
         exclude:
           - os: windows-latest
             node_version: 14
+          - os: macos-latest
+            node_version: 14
         include:
           - os: windows-2019
             node_version: 14
+          - os: macos-13
+            node_version: 14
     name: Node ${{ matrix.node_version }} on ${{ matrix.os }}
 
     steps: