cmake: move ZEPHYR_TOOLCHAIN_VARIANT export to common settings
cmake: move ZEPHYR_TOOLCHAIN_VARIANT export to common settings

Fixes: #43378

Move the exporting of ZEPHYR_TOOLCHAIN_VARIANT from the custom target to
the COMMON_KCONFIG_ENV_SETTINGS variable.
This ensures that the setting is exported both when running the initial
kconfiglib parsing but also on later menuconfig / guiconfig invocations.

Signed-off-by: Torsten Rasmussen <a7c5b1d5a1f41fce6edfa3f7c2392ae0c59deafe@nordicsemi.no>

galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr