feat: turn fatal error of missing user into warning

[tomberek]

Motivation

Turn the UID error into a warning.

Context

With the upcoming Sequoia update, UID's 301,302,303,304 become used by Apple-provided users. This causes a hard error. This PR intends to turn this into a warning, but to allow usage of Nix nevertheless.

This warning can get noisy if trying to do a lot of builds, this is not idea, but does allow usage.

Priorities and Process

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