riscv64 support

[techknowlogick]

I'm opening this ticket so that this request is tracked somewhere and made with no expectations/demands that it be completed or even looked into. Also, since riscv64 has limited support with nix itself, I expect adding riscv64 support in this installer would rely on that being better supported first.

More generally: I'm (very casually) poking at nix and riscv64 with hopes of having better support for this architecture, and if anyone else is interested, I'd be happy to share my wip as well as provide access to riscv64 hardware.