ci: ensure packages build successfully

Helps prevent broken URLs from breaking the build.

.github/workflows/ci.yml

@@ -7,6 +7,13 @@ on:
     branches: [ master ]
 
 jobs:
+  build:
+    name: nix build packages/*.nix
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: cachix/install-nix-action@v27
+      - run: basename -s .nix packages/* | sed 's/.*/.#&/' | xargs nix build
   test:
     name: nix flake check --all-systems
     runs-on: ubuntu-latest