x/sys/plan9, x/sys/windows: Environment functions should just refer to syscall functions

In x/sys/unix/env_unix.go, the functions that deal with the environment just call the functions in the standard syscall package.  The same should be done for x/sys/plan9/env_plan9.go and x/sys/windows/env_windows.go.  I would just do it but I don't have the systems to test on, and as far as I know the trybots will only test the main repository, not x/sys.
