fix(build): link bee-gpu-stress with -lm for sqrt()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/build.sh

@@ -180,7 +180,7 @@ if [ "$GPU_STRESS_NEED_BUILD" = "1" ]; then
         -I"${CUBLAS_CACHE}/include" \
         -o "$GPU_STRESS_BIN" \
         "${BUILDER_DIR}/bee-gpu-stress.c" \
-        -ldl
+        -ldl -lm
     echo "binary: $GPU_STRESS_BIN"
 else
     echo "=== bee-gpu-stress up to date, skipping build ==="