💡Use `TUNNEL_TOKEN` for systemd service so that TOKEN is not visible on `ps`

**Describe the feature you'd like**
Per title.

**Describe alternatives you've considered**
Are there any alternatives to solving this problem? If so, what was your experience with them?

**Additional context**
https://github.com/cloudflare/cloudflared/blob/f27418044ba359a17995166034d900c11aa50008/cmd/cloudflared/linux_service.go#L64

Can use `Environment=` directive.