modules/nixpkgs: don't assign elaborated platforms

[MattSturgeon]

See NixOS/nixpkgs#376988

Cherry-picked from #2964. There's no reason why we can't merge this early, to benefit users who have a nixpkgs revision ahead of ours.

Fixes #2967

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[MattSturgeon]

FYI: some parts of this PR will probably end up being reworked or reverted. The upstream change has been reverted for now, until it can be implemented in a less disruptive way. See also: NixOS/nixpkgs#376988 (comment).

The main change of not assigning an elaborated system to nixpkgs.*Platform options is a good change and will remain, regardless of how other parts of this PR are reworked.