ci: Build PRs and master branch (#17)

Using https://omnix.page/om/ci.html

Also remove website build, since we are not deploying anyway (domain
needs to be set).

.github/workflows/publish.yaml

@@ -4,23 +4,14 @@ on:
   push:
     branches:
       - master
+  pull_request:
 jobs:
   www:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.system }}
+    strategy:
+      matrix:
+        system: [x86_64-linux, aarch64-darwin]
+      fail-fast: false
     steps:
-      - uses: actions/checkout@v2
-      - uses: cachix/install-nix-action@v17
-        with:
-          extra_nix_config: |
-            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
-            trusted-public-keys = cache.srid.ca:8sQkbPrOIoXktIwI0OucQBXod2e9fDjjoEZWn8OXbdo= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
-            substituters = https://cache.srid.ca https://cache.nixos.org/
-      - name: Cache deps and build sources 🔧
-        run: |
-          nix build -j 4 .#site
-      - name: Deploy to gh-pages 🚀
-        uses: peaceiris/actions-gh-pages@v3
-        with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}
-          publish_dir: result
-          cname: nixpkgs.haskell.page
+      - uses: actions/checkout@v4
+      - run: om ci