MAINT: disable nb sync, pin julia=1.8+, add ghostcript

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[mmcky]

latexify needs additional dependencies

nbclient.exceptions.CellExecutionError: An error occurred while executing the following cell:
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# Creates algebraic symbols that can be used in an algebraic expression
@variables g, r, x0
G = (1 + g)
R = (1 + r)
p0 = x0 / (1 - G * R ^ (-1))
print("Our formula is")
latexify(p0) |> s -> render(s, MIME("image/png"))
------------------

failed process: Process(`lualatex --interaction=batchmode /tmp/jl_lsleTMhW16.tex`, ProcessExited(1)) [1]

[mmcky]

this has fixed the build issue with geom_series lecture

Will fix syncing with the notebook repository in a separate PR.