Extended CI

[angerman]

Our goal is to build and test on different systems. The test should run on a system closer to the end-user, not on the same we build the bindist on. While doing so gives us some very limited assurance it's not sufficient for high quality bindists.

@hasufell has build something similar in haskell/cabal#11083, and we should utilize the same.

We should have the follwing CI:

• (nix-ci.yml) nix provisioned ci for macos and linux as a baseline as we can get predictable provisioning this way and we already have this infrastructure built.
• (source-ci.yml) Source builds on a limited set of provisioned systems, we want to support people working and building GHC on.
• Tests run on a much larger matrix.

Crucially the tests should be independent of the actual bindist. Tests + bindist should give us enough to run the test-suite.