ci: clang: Do not run on third-party repositories
ci: clang: Do not run on third-party repositories

This commit updates the clang workflow to only run on the repositories
in the zephyrproject-rtos organisation because this workflow requires
a custom runner type only available within the zephyrproject-rtos
organisation.

Signed-off-by: Stephanos Ioannidis <dc76e9f0c0006e8f919e0c515c66dbba3982f785@stephanos.io>

galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr