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

Author: jfroche

nix/checks.nix

@@ -314,7 +314,9 @@
           // (import ./ext/tests {
             inherit self;
             inherit pkgs;
-          })
-        );
+        }))
+        // pkgs.lib.optionalAttrs (pkgs.stdenv.hostPlatform.isLinux) {
+          inherit (self'.packages) ansible-test run-testinfra docker-image-ubuntu;
+        };
     };
 }