.github/workflows: Run checks on 3 OSes.

This fixes #1670.

.github/workflows/ci.yml

@@ -131,7 +131,12 @@ jobs:
 
   checks:
     name: Run checks
-    runs-on: ubuntu-latest
+    # Most checks do not rely on the OS, but some (like check-gen) do. To ensure
+    # maximal coverage, we run all of the checks on the three main OSes.
+    strategy:
+      matrix:
+        os: [ubuntu, macos, windows]
+    runs-on: ${{ matrix.os }}-latest
     steps:
     - name: Checkout code
       uses: actions/checkout@v4