[RFC] ci: silence non-essential output

- Messages related to dependency resolution and third-party installs can
  be silenced without harming visibility in the build logs.
- But keep the messages for Neovim-related installs.
Maybe even use -qq for `apt-get update` (would get rid of ~100 lines)? This wouldn't show any output, though (it still should show output if something goes wrong, I think).