ci: remove redundant dry-build job

`just check-flake-outputs` does the same but in a more robust way, while
also checking devShells.

justfile

@@ -31,9 +31,7 @@ format:
   deadnix -eq $(git ls-files '*.nix')
   nixpkgs-fmt $(git ls-files '*.nix')
 
-ci:
-  #!/usr/bin/env sh
-  for config in $(nix flake show --json | jq -r '.nixosConfigurations | keys[]'); do
-    echo "Checking configuration: $config"
-    nixos-rebuild dry-build --flake '.#'"$config"
-  done
+check-flake-outputs:
+  nix flake check --all-systems --verbose
+
+ci: check-flake-outputs