scripts: west: runners: stm32cubeprogrammer: fix path lookup on macOS

On Windows and Linux, the stm32cubeprogrammer runner will use
STM32_Programmer_CLI from PATH if it is available, but this was not
implemented on macOS. Add PATH lookup for macOS as well, using the same
approach as Windows and Linux.

Fixes zephyrproject-rtos/zephyr#93056

Signed-off-by: Garrett L. Ward <glward@glward.net>