CI: Improve godot-cpp actions

[Repiteo]

Improves the GitHub Actions for godot-cpp in two primary ways:

• Relocated running C compiler on gdextension_interface.h to static_checks. This check was pushed waaaaay further back than it should've been, especially when the check itself takes less than a second. It's structurally and logistically much closer to the other static checks, which is where the action now resides.
• godot-cpp is now cached. There's no real reason for this SCons build to not have dedicated caching like the other workflows, so this adds exactly that. While this file would likely have to be retired entirely if other artifacts want to use godot-cpp, the current implementation means that the cache name can be safely hardcoded.

[AThousandShips]

Doesn't really affect anything but the godot-cpp build system doesn't have any cache limiting, so we'd have to see how this can be handled safely, but we do use the cache in godot-cpp so it shouldn't matter but just need to make sure it doesn't tax our CI too much by filling up our limits

[AThousandShips]

Note that just from this PR the cache upload size was 825 mb for a single run, that's a lot for the 10 gig cache limit, and might cause important other cache entries to be dropped and hurt general compile times

I think that, until we've implemented a cache limiting system in godot-cpp we should avoid using the cache here, especially since the build time is just 8 minutes at least in this build, compared to if we evict a linux build which is 30+ minutes without cache

[Repiteo]

Hmm, fair point; haven't considered any cache limiting techniques used here. For now, I'll keep those sections commented out and add a TODO note.

[AThousandShips]

I'll take a look at adding cache pruning to godot-cpp over the next few days and we can see how it works out

[dsnopek]

Looks good to me!

[akien-mga]

Thanks!