Add GitHub workflow for building+installing into toolchain via CMake.

[dan-zheng]

Add GitHub Actions workflow for:

• Downloading and installing a swift.org/download macOS development snapshot
• Building tensorflow/swift-apis via CMake
• Installing tensorflow/swift-apis into the toolchain via CMake
• Repackaging the toolchain into a .pkg
• Publishing the toolchain .pkg artifact

This allows users to download standard swift.org/download toolchains with TensorFlow and X10 installed.

[dan-zheng]

The repackaged toolchains built as part of this GitHub action are currently not right (way too small, much is missing). I'm not sure why, would be good to investigate. This action is still useful for testing CMake-based builds and installs.