Fix broken build

What does this MR do?

This MR alters the base image for license-finder to rely on debian:buster over debian:stable.

The first attempt was to bump omnibus as one of the changes in the new debian version changed the signature of a file handler call: https://github.com/chef/omnibus/commit/dc458d0de3619e1e82da30c4f8cba3531ff24ba4

Example pipeline: https://gitlab.com/gitlab-org/security-products/analyzers/license-finder/-/pipelines/354797921

However, this only partially fixed the issue as then there were several problems with the python builds around missing libraries which omnibus loads dynamically: https://www.debian.org/releases/bullseye/amd64/release-notes/ch-information.en.html#nss-nis

Example pipeline: https://gitlab.com/gitlab-org/security-products/analyzers/license-finder/-/pipelines/355426379

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added