nixos: use uniq in the type of system.build

[ncfavier]

unspecified will happily concatenate strings together from two unrelated modules, causing spurious errors (see #155925).

Ideally we could make system.build a submodule with a few distinguished options like installBootLoader, but I don't think this is necessary right now.

I've tested this by running a vm with build-vm-with-bootloader.

[roberth]

I didn't know that it did that, but anything also merges, deeply even. How about uniq unspecified?

Regarding the submodule, see blocked pr #155883.

[ncfavier]

Sounds good, I looked at all definitions of system.build.* and couldn't find anything that relies on merging.

[ncfavier]

@ofborg test grub systemd-boot login

[roberth]

Before attrsOf unspecified, it was attrs, which would replace attributes arbitrarily in case of duplicates; arbitrary being determined by the module import order and module system internals. So uniq will only find pre-existing problems.

[roberth]

OfBorg evaluates fine. The error on arm is caused by a build failure after eval, caused by a full disk.

I hope not too many collisions exist out there, because they're about to be discovered -- well, except for the fraction of users that turned away because they couldn't figure out why their config or module didn't apply because of the silent collisions. Sad.

Let's fix this problem for current and future configs. Merging.