* build.zig: remove detour through zig0. I'll add it back as an option
   later. This means you can build stage1 with a freshly built zig
   without detouring through zig0 again.
 * remove unused file windows_script.bat
 * drone.yml: remove unnecessary steps
 * ninja doesn't need a jobs parameter
 * build the release version of zig with zig build instead of cmake.
 * build stage2 with -Dstatic-llvm -Duse-zig-libcxx