CI: Improve GTA with matrix tests on X84 and ARM: shell x image

[tinmarino]

I worked a little on github actions for them to use a matrix strategy.

The ARM file is still independant as it is using another remote action (it would be nice to have CPU in a matrix field but I did not take the time to make this "conditional use of subaction").

It looks like that on my repo, also see the Action output. But the last commit is changing the image URL to point to this root repo => ready to merge.

[tinmarino]

Sorry for the force push, I had to rebase on this repo main to separate with my other PR.

It seems that DDexec is not working fine on ARM At least the tests are not happy.