Create a Central Repository Config File

[reneme]

Some configurations are duplicated multiple times in the repository: I had said we should centralize some of those.

Suggestion:

Lets add a file in src/configs/repo_config.env that can contain such configurations. Side note: Personally, I'd probably add it as a root-level file (eg. .repo_config.env), but I remember that you wanted to keep the repo root as tidy as possible.

This file could then be consumed by .github/workflows/ci.yml and .../nightly.yml as well as here and (for now) just contain:

# the fork of boringssl that should be used for BoGo tests
BORINGSSL_REPO=https://github.com/randombit/boringssl.git

# the branch in our fork of boringssl that should be used for BoGo tests
BORINGSSL_BRANCH=rene/runner-20241016

We might have more such configuration that is worth to centralize like that. In any case: I'd do this adaption in an independent PR (likely after 3.6.0).

What are your thoughts on this?

Originally posted by @reneme in https://github.com/randombit/botan/pull/4389#discussion_r1804636865