How to set custom pihole.toml path?

How to set custom pihole.toml path? It would be very useful for NixOS users.

NixOS is not one of Pi-hole's Supported Operating Systems.
Is this a request to support NixOS?

The location of pihole.toml isn't configurable.

Still, you could opt to run Pi-hole as a Docker container under NixOS.
If you would, you could provide all configuration options via Docker environment variables.