Add `poetry` to build image included software

I believe this was raised on the GitHub as well, but since the build-image GitHub repo has been archived I thought I would raise it again here.

The poetry package manager is quite popular within the Python community and it would be really nice to have it be part of the software included in the build image. I realise only a small fraction of Netlify builds use Python as their primary language, but I suspect a fair number might be using it as part of their build tooling, which is indeed the case for me.

Hi @JeppeKlitgaard :wave:t6: ,

Thanks so much for reaching out and welcome back to the forums! I have passed your feedback along to the product team. Thanks so much for sharing.