* Fix Ruby Windows 3.1 (x64-mingw-ucrt) build:
- Pass-in CPPFLAG "GPR_WINDOWS_UCRT" to pickup correct shared object file.
- Rename intermediate legacy SO file from 'grpc_c.64.ruby' to 'grpc_c.64-msvcrt.ruby' for clarity.
- Add artifact target for Ruby x64-mingw-ucrt
* Change CPP flag name