GitHub Workflow: Use new setup-godot-cpp github action from godot-cpp submodule.

[Ivorforce]

Follow up of #68.

With this PR, the GH actions are changed to use setup-godot-cpp from the godot-cpp repository (local version). This lifts the same burden from this repository, making it more future proof and easier to maintain.

[Ivorforce]

Sorry for the repeated pushes... Somehow, I got stuck with git issues regarding the submodule. I simply don't understand those things...

Anyway, the PR is updated to use the latest godot-cpp commit now. It should still run through. But we still shouldn't merge because that would upgrade the repo to godot 4.x. Should merge until the change is cherry-picked to 4.3 at least.

[Ivorforce]

I updated godot-cpp to f3a1a2fd458dfaf4de08c906f22a2fe9e924b16f (4.3 branch). It should be mergable now!

Edit: As promised earlier, I also updated the readme to reflect the new action. I think it is better to document its structure in the file itself (lest we have its content twice), so I removed the copy from README.md added some comments in builds.yml. Open to adding more comments!

[paddy-exe]

Looks good to me! Thanks!

[Ughuuu]

Ok, this once again broke my builds, since it deleted the build action. I guess I will write it manually for all my repos since it changes so often and breaks compatibility.

[Ivorforce]

@Ughuuu This PR actually addresses that exact issue :)
The godot-cpp repo has a setup-godot-cpp github action now. I would recommend you adjust your workflow the exact same way as this PR does to the template workflow. This should make your workflow future proof, and stop breaking your builds.

[Ughuuu]

I see the idea, but not rlly, it's not versioned still, no?
./godot-cpp/.github/actions/setup-godot-cpp
it's not @v1 or so on..

[Ivorforce]

Since the new version is using a local action (in ./godot-cpp), it won't change unexpectedly as long as you don't update godot-cpp. Versioning isn't required.