It has happened repeatedly that C++ code accidentally crept into the
GDExtension C header, a notable example being use of the `bool` type.
This change adds a CI step to check the header with a C-only compiler.
The min SCons version had to be bumped as SCons 3.0 before 3.0.3 seems
broken (see #92043), and there's little gain from supporting 3.0.3-3.0.5.
3.1.2 is also the first version to avoid ambiguities between Python 2
and Python 3 usage, so we finally use it as the minimum baseline.
Also test against Python 3.6 which is also our minimum supported version.
This should help prevent regressions whenever we modernize the build scripts.
This ensures that the godot-cpp job has plenty of resources
to run its build and avoid being affected by the main build.
Additionally:
- Extract test tasks into dedicated actions.
- Upload artifacts as early as possible.
- Ensure that we check master cache before random cache.
