ci: run main tests on macOS and Windows, too

.github/workflows/check.yml

@@ -8,13 +8,19 @@ on:
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-latest, ubuntu-latest, windows-latest]
     steps:
       - uses: actions/checkout@v4
       # No caching as we're expected to run rarely
       - name: Install dependencies and run checks
+        shell: bash
         run: |
           set -euxo pipefail
+          python3 -m venv venv
+          source venv/bin/activate
           python3 -m pip install pre-commit
           export PRE_COMMIT_COLOR=always
           pre-commit try-repo . --files .github/workflows/test-good.sh