cmake loads native sysroot cmake config and tries to look for libs

Integrating https://codereview.qt-project.org/c/yocto/meta-qt6/+/413010/4 ends
with weird issue of target cmake pulling native cmake files for resolving zlib.

As change introduces system zlib and native sysroot qtbase was configured without it ends up in error. However target cmake configuration should not even try to find packages in native sysroot for libs in first place.

https://testresults.qt.io/logs/yocto/meta-qt6/09249dcbe4d6e41a0bbf9c2f9b939de53e821c48/LinuxUbuntu_20_04x86_64LinuxUbuntu_20_04target1Yoctoqtci-linux-Ubuntu-20.04-x86_64-50-00a1d6/9e7362e5587211f7ef9e96e768c6d42a7e79fc52/build_1654874601/log.txt.gz