test Nix Flake on both Mac and Linux

Author: peterbecich

.github/workflows/nix-flake.yaml

@@ -3,8 +3,15 @@ on:
   pull_request:
   push:
 jobs:
-  tests:
-    runs-on: ubuntu-latest
+  nix:
+    strategy:
+      fail-fast: true
+      matrix:
+        os:
+        - ubuntu-latest
+        - macos-latest
+    name: Nix on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main