doc: gsg: Include instructions to set SDK env vars

The basic environment variables need to be set to be able to build
blinky (at least ZEPHYR_TOOLCHAIN_VARIANT) but were not inlcuded in the
GSG.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
Co-authored-by: Marti Bolivar <marti.bolivar@nordicsemi.no>