The old way of constructing FLASH_SCRIPT_ENV_VARS was corrupting the
values that were passed to the flasher. This new method is the
standard way of creating a dictionary/hashmap in CMake and does not
suffer from the same problem.
This fixes
https://github.com/zephyrproject-rtos/zephyr/issues/4844#event-1334599401
Signed-off-by: Sebastian Boe <sebastian.boe@nordicsemi.no>