ci: make sure we cache system manager and its test dependencies

Author: jfroche

nix/checks.nix

@@ -314,6 +314,9 @@
             inherit pkgs;
           };
           devShell = self'.devShells.default;
+        }
+        // lib.optionalAttrs (pkgs.stdenv.hostPlatform.isLinux) {
+          inherit (self'.packages) ansible-test run-testinfra docker-image-ubuntu;
         };
     };
 }