bootstrap-conda: Do not include sage in src/environment-optional.yml #35587
Description
The failures in "Build & Test using Conda" for "conda ... environment-optional" (#35555 (comment)) are from including our dummy package _sagemath.
This also affects users who try to follow our instructions at https://doc.sagemath.org/html/en/installation/conda.html#using-conda-to-provide-system-packages-for-the-sage-distribution