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 <Torsten.Rasmussen@nordicsemi.no>