.cachix.dhall: add ampersand

.cachix.dhall

@@ -10,7 +10,7 @@ let H =
 let toStep
     : Text → Text
     = λ(x : Text) →
-        "nix build --json .#${x} | jq -r '.[].outputs | to_entries[].value' | cachix push horizon --accept-flake-config"
+        "nix build --json .#${x} | jq -r '.[].outputs | to_entries[].value' | cachix push horizon --accept-flake-config &"
 
 let input =
       Prelude.Map.keys