CI: test on macOS/{Intel,M1}

The macOS 14 runner is an M1 machine
according to:

https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

so use that and macOS 13 to get an Intel
runner.

.github/workflows/build.yml

@@ -18,7 +18,7 @@ jobs:
       # Do not cancel other jobs on first failure.
       fail-fast: false
       matrix:
-        os: [ ubuntu-latest, macos-latest ]
+        os: [ ubuntu-latest, macos-13, macos-14 ]
     runs-on: ${{ matrix.os }}
 
     steps: