Choose `paths` based on environment variable

[slowmoyang]

Switching the default paths preset to khu hard-codes an environment-specific data_dir as the global default for tts. This will break runs on other machines unless they remember to override it. Consider keeping paths: default and sourcing data_dir from an env var (or leaving it unset with a clear error), and/or documenting/overriding paths=khu only for lapras.lo.

  - paths: default

Originally posted by @Copilot in #43 (comment)