`name` does not exist.  I think the `environment` section of the configuration
tricked me.

Just use the configured temporary directory in the environment.  This is less
great but it's still okay because:

* It's our system
* The deploy key has limited capabilities

docs.nix used to use <nixpkgs> so maybe that was why

Using a copy of the docs artifacts for packing up
should not exhibit the problems from earlier.

See https://docs.gitlab.com/ee/ci/environments/