Add nix build CI workflow

Author: InfinityGhost

.github/workflows/nix.yml

@@ -0,0 +1,17 @@
+name: nix build
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v18
+      with:
+        extra_nix_config: |
+          access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
+    - run: nix build
+    - run: nix flake check

flake.nix

@@ -21,7 +21,6 @@
       default = tabletbot;
     };
 
-    devShell.${system} = import ./shell.nix { inherit pkgs; };
-
+    devShells.${system}.default = import ./shell.nix { inherit pkgs; };
   };
 }