cmake: Use cmake var not env for sdk

ZEPHYR_SDK_INSTALL_DIR is set from the env if it is not passed
explicitly to cmake, but one usage was reading directly from the env,
not allowing passing the var to cmake instead.

Change find_package of Zephyr-sdk to read from cmake var not env var.

Signed-off-by: Jeremy Bettis <jbettis@google.com>