CI: Run it in a nix develop shell

.forgejo/workflows/ci.yaml

@@ -9,10 +9,6 @@ jobs:
     steps:
       - uses: actions/checkout@v3
         name: Checkout
-      - name: Set up Cachix
-        run: |
-            nix-env -iA nixpkgs.cachix nixpkgs.bash
-            cachix use $CACHIX_NAME
       - name: Build
         run: |
-          cachix watch-exec $CACHIX_NAME nix build
+          nix develop --command bash --command 'cachix use $CACHIX_NAME && cachix watch-exec $CACHIX_NAME nix build'

flake.nix

@@ -22,7 +22,7 @@
 
         # For `nix develop`:
         devShell = pkgs.mkShell {
-          nativeBuildInputs = with pkgs; [ rustc cargo ];
+          nativeBuildInputs = with pkgs; [ rustc cargo cachix ];
         };
       }
     );