[Docs] `generate-lua-library` workflow will push changes to wrong branch on forks

Example: https://github.com/SlashScreen/RecoilEngine/actions/runs/14849763696/job/41691057982

If `RECOIL_LUA_LIBRARY_RESPOSITORY` is not set, then the [checkout action](https://github.com/beyond-all-reason/RecoilEngine/blob/0aa5469497c42b9066a304f13f76ea460aa69b07/.github/workflows/generate-lua-library.yml#L35-L40) will default to current repo, resulting in the library being pushed locally.

Start with .github/workflows/generate-lua-library.yml, especially the checkout action at lines 35-40, and review the linked workflow run from a fork. Confirm the repository and branch used when RECOIL_LUA_LIBRARY_RESPOSITORY is unset; done means the workflow targets the intended branch rather than pushing the generated library locally.

Written by the indexing model from the issue text.