The Github action should not require a full build

[JohnVidler]

Currently, the build script downloads all dependencies, then compiles a blank application, and finally builds the documentation.

The compile phase is an unnecessary step, and just slows down the build, although it only happens once when testing locally as the binaries are cached anyway.

To fix, we'd need a 'download only' build phase, which does not attempt to run the actual arm build, then we can skip the compiler pull stage and the source build before moving on to documentation build steps directly.