NixOS is not reproducible

NixOS is not reproducible.
Okay, sorry for the clickbait.
NixOS is not reproducible according to the Reproducible Builds definition.
I keep reading people making this claim repeatedly on orange-site, even made a similar claim when writing about Nix and Guix earlier this week.1 Along with their recently launched wiki.
So, what is the Reproducible Builds definition?2
When is a build reproducible?
A build is reproducible if given the same source code, build environment and build instructions, any party can recreate bit-by-bit identical copies of all specified artifacts.

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