De-duplicate GitHub actions config

Author: dstansby

.github/workflows/ci-osx.yaml

@@ -1,4 +1,4 @@
-name: Linux CI
+name: Tests
 
 on: [push, pull_request]
 
@@ -8,11 +8,12 @@ concurrency:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.platform }}
     strategy:
       fail-fast: false
       matrix:
         python-version: ["3.8", "3.9", "3.10", "3.11", "3.12"]
+        platform: [ubuntu-latest, windows-latest, macos-12]
 
     steps:
       - name: Checkout source
@@ -41,6 +42,13 @@ jobs:
           c-compiler cxx-compiler
           python=${{matrix.python-version}} wheel pip
 
+      - name: Install clang
+        shell: "bash -l {0}"
+        if: ${{ matrix.platform == 'macos-12' }}
+        run: |
+          conda activate env
+          conda install -y 'clang>=12.0.1,<17'
+
       - name: Show info about `env` environment
         shell: "bash -l {0}"
         run: |