Generated `nix.conf` is not deterministic about nix-installer

[emilazy]

I should note also that the order is not completely irrelevant:

A configuration setting usually overrides any previous value. However, you can prefix the name of the setting by extra- to append to the previous value.
— https://nixos.org/manual/nix/stable/command-ref/conf-file.html

but I don't know if this is relevant to nix-installer.

[Hoverbear]

This is unfortunately a consequence of our nix.conf merging behavior. It's most certainly a place we could improve. We should probably be preserving the ordering/format where possible.

[emilazy]

FWIW, only the order of the file on a clean install really matters for us, since it's just a matter of having a consistent hash. Anyone doing customization pre-install hopefully knows what they're doing already and can add the hash or manually move the file away. So just removing the nondeterminism from the added options would be adequate for us, even though it might be worth considering ordering in general due to the odd behaviour of the extra-* options.