fix: ci: leave cargo's C compiler to the `cc` crate on Windows

The Windows job sets `CXX` to msys2's `c++` for CMake, but cargo there targets MSVC, and `con-ron`'s `mimalloc` dependency compiles C through the `cc` crate, which took that `CXX` and failed. Without it, the crate finds `cl.exe` itself; `nanoda` has no C to compile and never noticed.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WoCksQxcFUG88JcjzFe4bM