Exclude cgotest from stdlib build

[dzbarsky]

It didn't build for me with a zig toolchain, but anyway we shouldn't need to build this for user code

# net/internal/cgotest
bazel-out/darwin_arm64-opt-exec-ST-6c94b0e707c0/bin/external/rules_go+/stdlib_/src/net/internal/cgotest/resstate.go:10:10: fatal error: 'resolv.h' file not found
   10 | #include <resolv.h>

What type of PR is this?
Perf tweak?

[linzhp]

This only happen on macOS, right? Zig toolchain cannot build for macOS right now. Can we only exclude this tests for macOS builds?