ci: do_not_merge: Use Ubuntu 20.04 runner image
ci: do_not_merge: Use Ubuntu 20.04 runner image

This commit updates the "Do Not Merge" workflow to use a specific
runner image version, ubuntu-20.04, instead of the latest version in
order to prevent any potential breakages due to the 'latest' version
change by GitHub.

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

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