We now assume the daemon is used unconditionally when we manage the
Nix installation.
The `nix.gc` and `nix.optimise` services lose their `$NIX_REMOTE`
setting rather than making it unconditional, as the NixOS `nix.gc`
module does not set it. Possibly it should, but I think uniformity
between the two systems is better than diverging, even though I kind
of hate that the non‐daemon method of access is even a thing.
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