After a recent CI image builder update successfully ran the tests, but
didn't run the cross tests, it updated the image that then got used by
the regular CI cross tests. Somehow this resulted in a bunch of tests
now failing because zlib could not be picked up. We probably dropped a
transitive dependency somewhere. Anyway, it's correct to explicitly
specify it if we need it.
We've never used it for anything, it was originally added for #3776 but
that never got finished so it's just a waste.
This also prevents successful regeneration of the build image, because
nim is not available for Ubuntu rolling. It's available in 20.04 and
22.10, but vanished in between for reasons best known to Ubuntu.
This has been removed as an explicit package in impish. It seems that
having pkg-config installed and adding arm as an arch will cause it to
be generated automatically