Add a GHA workflow to test our CMake build

### Description #1523 was recently merged, then [broke the macOS trunk snapshot toolchain CI](https://ci.swift.org/job/oss-swift-package-macos/4813/console), because it was using macros that aren't there yet when building the toolchain, which had to be fixed in #1609. This probably wasn't detected during PR testing because all the current PR CI jobs are using the macros from a prebuilt toolchain instead. ### Reproduction Do what the linked CI job does, ie build fresh macOS toolchain using macros as in #1523 ### Expected behavior CI to catch such regressions ### Environment macOS ### Additional information It may be possible to set up a PR CI job with the prebuilt nightly snapshot toolchain and `build-script`, by passing in the prebuilt toolchain using the `--skip-build-swift --native-swift-tools-path` flags and building and installing this repo and its macros using that script alone, as the toolchain CI does in fuller form after a bunch of pulls are merged every day. This would warn about such build errors _before_ such pulls are merged.