TSL: Avoid side effect of getTextureLevel() in normals.

[Mugen87]

Fixed #32870.

Description

As suggested in #32875 (comment), this PR makes sure the Normal node is not affected by getTextureLevel() context definitions from e.g. EnvironmentNode.

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

	Before	After	Diff
WebGL	357.49 84.84	357.49 84.84	+0 B +0 B
WebGPU	622.71 172.91	622.75 173	+42 B +87 B
WebGPU Nodes	621.31 172.67	621.35 172.76	+42 B +90 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

	Before	After	Diff
WebGL	490.47 119.9	490.47 119.9	+0 B +0 B
WebGPU	693.82 188.21	693.86 188.21	+42 B +5 B
WebGPU Nodes	643.2 175.31	643.24 175.31	+42 B +4 B