How should I mention on my CV that I will leave my current PhD program when I apply to a new one?
Your CV is not the right place to mention this. Instead, you should write a brief explanation near the beginning of your personal statement or cover letter which explains your reasons for applying to a new PhD. I would advise that you think carefully about how you explain your motivations. If you sound like an uncommitted student (even if that is down to your environment rather than yourself), a new supervisor or department are unlikely to want to take you on.
Make sure that your CV is an honest record of your past employment and education. You should not omit your current position as a PhD student, even if you aren't really working for it.