github: hello_world_multiplatform: run on Ubuntu ARM

Add ubuntu-24.04-arm to the list of runners, while this runner is still
flagged as being in beta, it's been around for more than one year and
has been running for quite a long time in the action-zephyr-setup
repository with no problems.

Adding it to the list so we get some coverage of the Linux/ARM toolchain
and setup, easy enough to get rid of it if it causes problems.

Link: https://github.com/actions/partner-runner-images/
Link: https://github.blog/changelog/2024-06-24-github-actions-ubuntu-24-04-image-now-available-for-arm64-runners/


Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>