This is a huge anti‐declarative footgun; `copy` files cannot
distinguish if a previous version is managed by nix-darwin, so they
can’t check the hash, so they’re prone to destroying data, and
copied files are not deleted when they’re removed from the system
configuration, which led to a security bug. Nothing else in‐tree
was using this functionality, so let’s make sure it doesn’t
cause any more bugs.
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