Move CMake CI for this repo to be publicly visible

The project is tested continuously with CMake on Kokoro, but this is invisible outside of Google right now.
It would be great if this CI was part of the publicly visible Kokoro.

To begin with, what about updating copybara to publish in the repo the build script used by the Kokoro CI right now to build with CMake?

See also: https://github.com/llvm/torch-mlir/issues/1835 