Previously main.go used syscall.SysProcAttr{Setpgid: true} and
syscall.Kill, both undefined on Windows. This broke GOOS=windows
cross-compilation.
Add a //go:build !windows constraint to the package since it is
a dev-only tool that requires Unix utilities (bash, make, etc.)
and is not intended to run on Windows.
Refs #23054Fixescoder/internal#1407