Only run on PR. Split up macos build by python version

.github/workflows/build_wheels.yml

@@ -1,6 +1,6 @@
 name: Build
 
-on: [push, pull_request]
+on: [pull_request]
 
 jobs:
   build_wheels_macos:
@@ -10,6 +10,7 @@ jobs:
       matrix:
         # macos-13 is an intel runner, macos-14 is apple silicon
         os: [macos-13, macos-14]
+        py: [cp38, cp39, cp311, cp312]
 
     steps:
       - uses: actions/checkout@v4
@@ -25,7 +26,8 @@ jobs:
         env:
           # configure cibuildwheel to build native archs ('auto'), and some
           # emulated ones
-          CIBW_ARCHS_LINUX: auto aarch64 ppc64le s390x
+          CIBW_BUILD: ${{ matrix.py }}-*
+          CIBW_ARCHS_LINUX: auto
 
       - uses: actions/upload-artifact@v4
         with: