Write me a Python script that uses the AI Builder Space speech recognition API to transcribe the voice memos in the memos/ folder to text, and saves each one as a .txt file with the same name in transcripts/. The API key is in the environment variable AI_BUILDER_TOKEN. You are working in a folder: you can read files, run commands, and look at the results.

(Rules: read the API key only from the environment variable AI_BUILDER_TOKEN; never write it to any file and never print it. Work only inside the current folder.)