ci: Test on macOS as well

Although this library is written in pure Nix, the built-in support for regular
expressions is platform-dependent, which can cause breakage when run on
different platforms. It's safer to test on all platforms.

.github/workflows/test.yml

@@ -6,7 +6,12 @@ on:
   workflow_dispatch:
 jobs:
   check:
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os:
+          - ubuntu-latest
+          - macos-latest
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
       - uses: cachix/install-nix-action@V28