Fix scene_dataset being overwritten by episode config

[Functionhx]

Fixes #2090

Problem

In Env.__init__, the simulator's scene_dataset was unconditionally overwritten with the current episode's scene_dataset_config. For HM3D InstanceImageNav and other semantic tasks, this broke semantics loading because the episode data carries a basic scene dataset config (e.g., "default" or hm3d_basis.scene_dataset_config.json) that lacks semantic annotation paths, overriding the user's semantic-aware config (e.g., hm3d_annotated_basis.scene_dataset_config.json).

Root Cause

The config's simulator.scene_dataset defaults to "default" (from default_structured_configs.py). The episode's scene_dataset_config also defaults to "default" (from dataset.py). When a user explicitly sets scene_dataset to a semantic config, the episode loading code would blindly overwrite it, causing the simulator to initialize without semantic annotations.

The resulting error manifests as warnings like:

The active scene does not contain semantic annotations : activeSemanticSceneID_ = 0

Fix

Only use the episode's scene_dataset_config as a fallback when the simulator config's scene_dataset is still set to its default value ("default"), indicating the user hasn't explicitly configured it. When the user provides an explicit scene_dataset path, that choice is now respected.

Change

habitat-lab/habitat/core/env.py: Added a guard if self._config.simulator.scene_dataset == "default": before overriding with the episode's config.

Testing

This is a minimal, targeted fix. The existing behavior is preserved when scene_dataset is "default" (no user override). The only change is that an explicitly configured scene_dataset is now respected instead of being silently overwritten.