oe-find-native-sysroot: don't clear OECORE_NATIVE_SYSROOT
The OECORE_NATIVE_SYSROOT may come from environment vars, so don't clear
it, otherwise it broke runqemu-extract-sdk on sdk.
[YOCTO #11409]
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>