TSL: Fix `getBitangent()`. (#31059)

* TSL: Fix `getBitangent()`.

* E2E: Update screenshot.

src/nodes/accessors/Bitangent.js

@@ -19,7 +19,7 @@ const getBitangent = /*@__PURE__*/ Fn( ( [ crossNormalTangent, varyingName ], bu
 
 	if ( builder.material.flatShading !== true ) {
 
-		bitangent = varying( crossNormalTangent, varyingName );
+		bitangent = varying( bitangent, varyingName );
 
 	}