nixos/knot-resolver: skip config validation when cross-compiling
Since this module allows for customizing the manager package (which is
also used for config validation) as opposed to pulling the
build-platform compatible variant from the package-set, we cannot
assume this package is capable of being executed on the build
platform.
(cherry picked from commit d8cb98b5156011db1ceb4773ed4713c91804d4bc)