ci(macos): s/macos-latest/macos-13/

Workaround-for: actions/runner-images#9663

Author: erikmd

.github/workflows/build-macos.yml

@@ -14,7 +14,7 @@ on:
 jobs:
   build:
     name: Build learn-ocaml on macOS
-    runs-on: macos-latest
+    runs-on: macos-13
     steps:
       - name: Check out the repo
         uses: actions/checkout@v3