pnnx ci pin safetensors (#6236)

Author: nihui

.github/workflows/pnnx.yml

@@ -316,7 +316,7 @@ jobs:
         pip3 install --user pytest wheel twine requests einops numpy==${{ matrix.numpy }} opencv-python==${{ matrix.opencv }}
         pip3 install --user torch==${{ matrix.torch }}+cpu torchvision==${{ matrix.torchvision }}+cpu torchaudio==${{ matrix.torchaudio }} --index-url https://download.pytorch.org/whl/cpu
         pip3 install --user onnx onnxscript
-        pip3 install --user "transformers<=4.52.1" diffusers
+        pip3 install --user "transformers<=4.52.1" diffusers "safetensors<=0.6.0"
 
     - name: setup-pytorch-execstack-or-patchelf
       if: ${{ matrix.python }} == '3.8' || ${{ matrix.python }} == '3.9'