nix: clarify config.nix hack
nix: clarify config.nix hack

The comment said that this was needed for Nix <2.3.8, which is wrong
-- it's needed for all 2.3.x.  I think this must have been caused by a
nmisreading of the conditional, which applied to all versions _later_
than 2.3.8, not earlier.

But we don't package anything that old any more, so we can remove that
check entirely, in addition to clarifying the comment.

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