ci: Build and cache packages for macOS via GitHub Actions

After spending a massive amount of time making RetroArch build on macOS
(github:NixOS/nixpkgs#102230), I've decided to make sure my packages
have the right platforms on their `meta.platforms` and `meta.broken`.

Looks like builds.sr.ht has FreeBSD images.  Might try to build for that
later.

.github/workflows/darwin.yml

@@ -0,0 +1,20 @@
+name: "macOS"
+on:
+  push:
+jobs:
+  build:
+    strategy:
+      matrix:
+        platform: ["macos-latest"]
+        channel: ["nixpkgs-unstable"]
+    runs-on: "${{ matrix.platform }}"
+    steps:
+      - uses: actions/checkout@v2
+      - uses: cachix/install-nix-action@v12
+        with:
+          nix_path: "nixpkgs=channel:${{ matrix.channel }}"
+      - uses: cachix/cachix-action@v7
+        with:
+          name: "aasg-nixexprs"
+          signingKey: "${{ secrets.CACHIX_SIGNING_KEY }}"
+      - run: "nix-build --no-out-link --keep-going --show-trace ./ci.nix"