add GPU and distributed build CI

[TysonRayJones]

Running the unit tests distributed and GPU-accelerated will require our own hosting, and is a somewhat large task. However, merely compiling for those backends to at least detect build errors can be done right away, in the same fashion as the MSVC build test.

[TysonRayJones]

As of June, Github Actions can (in exchange for goods and services $$$) host single-GPU CI

https://github.blog/changelog/2024-07-08-github-actions-gpu-hosted-runners-are-now-generally-available/