chore(build): Use significantly fewer layers for Nixery itself

Nixery itself is built with the buildLayeredImage system, which takes
some time to create large numbers of layers.

This adjusts the default number of image layers from 96 to 20.

Additionally Nixery's image is often loaded with `docker load -i`,
which ignores layer cache hits anyways.

Additionaly the CI build is configured to use only 1, which speeds up
CI runs.

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