ci.yml: Build meson on darwin

We're building a bit of Darwin meson indirectly through `checks`,
but it'd be annoying to encounter broken un-`check`-ed stuff
during the porting process, so let's just do the right thing now.

.github/workflows/ci.yml

@@ -195,7 +195,11 @@ jobs:
       - run: nix build -L .#hydraJobs.tests.githubFlakes .#hydraJobs.tests.tarballFlakes .#hydraJobs.tests.functional_user
 
   meson_build:
-    runs-on: ubuntu-22.04
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main