boards: nucleo_l073rz: Add stm32cubeprogrammer as runner

stm32cubeprogrammer is much faster to flash than openocd.
Additionally, it can flash even when board is in stop mode,
which is required to test power management in CI.

Set it as a flasher of the board, so we can take advantage of it in CI.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>