Split Gitlab CI caches per job

A single cache for all Python versions-specific Conda environments is now too big, causing out-of-disk-space related errors (here and here). Split the caches per job, so we keep the environments separate, resulting in smaller caches. This should also speed up all jobs - especially those that don't need the cache (build sdist, docker, release).

The cache keys now have a version number (starting at "v1") to allow easy invalidating of the cache. We can just increment the number when necessary.

Added a clean step for Conda environments to further reduce size (removes index cache, lock files, unused cache packages, and tarballs).