ci: remove broken docker cache step (#5778)
Removes a broken docker cache step. I'm not sure if it ever worked, all
recent actions report `Failed to restore: Cache service responded with
400`. The `Post Cache Docker images` step also causes [CI failures for
some images
](https://github.com/prisma/prisma-engines/actions/runs/22143613049/job/64014401975?pr=5777)when
it runs `docker save`.
