ci: twister: Use Ubuntu 20.04 runner image
ci: twister: Use Ubuntu 20.04 runner image

This commit updates the "Run tests with twister" 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,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr