GitLab CI: pkg-config missing in build environment

[chreekat]

Describe the bug
When a dependency package of cabal-install (or cabal-install itself) introduces a dependency on pkg-config, building in GitLab CI will fail. Example: The pipeline started failing in Feb 2024 because an updated zlib uses pkg-config by default. (First failed pipeline: https://gitlab.haskell.org/haskell/cabal/-/pipelines/90520)

To Reproduce
Steps to reproduce the behavior:

1. Make cabal-install or one of its dependencies use pkg-config
2. Build cabal-install in GitLab CI on affected CI runners and/or Docker images

Additional context
NB: GitLab CI is not the same as GitHub CI.

The example given of zlib is being fixed separately. This issue tracks the larger task of actually providing pkg-config for zlib, or any other package that wants it. The quickest way of doing this is not the best way, I think.