fix(core): use from_env for sandboxed apps

[setoelkahfi]

This pull request updates how the Hugging Face Hub API client is initialized throughout the codebase, switching from direct construction with Api::new() and ApiBuilder::new() to using ApiBuilder::from_env(). This change ensures that environment variables are consistently respected when building the API client, improving configuration reliability.

Hugging Face Hub API Initialization

• Replaced direct instantiation of the API client (Api::new()) with ApiBuilder::from_env().build() in FluxStepper::new to ensure environment variables are used for configuration.
• Updated DiffusionLoader to use ApiBuilder::from_env() instead of ApiBuilder::new(), further standardizing API client initialization.
• Updated imports to include ApiBuilder in addition to Api and ApiError in stepper.rs.