Automatically enable pre-builds for end-users

[lostintangent]

This repo has pre-builds enabled, but that would only apply to folks that are contributing back to the template. In order to cache the pip install step for all end-users, it would be ideal if we could enable pre-builds for the template, and have this automatically accrue to folks that simply want to play with LangChain (and don't want a repo).

This behavior already exists, if a user selects Open in a Codespace vs. Create a new repository. However, the Open in a Codespace option doesn't support secrets, and therefore, it's not usable with this sample (which requires an OpenAI key). Therefore, we need Codespaces to support secrets with "repoless" Codespaces, before we can make pre-builds "just work".