Update gluon_nlp_job.sh (#1576)

Author: barry-jin

tools/docker/gluon_nlp_job.sh

@@ -24,7 +24,7 @@ git checkout working
 
 if [ $DEVICE == "cpu" ]; then
   python3 -m pip uninstall --quiet mxnet -y
-  python3 -m pip install -U --quiet --pre "mxnet>=2.0.0b20210121" --user
+  python3 -m pip install -U --quiet --pre "mxnet>=2.0.0b20210121" -f https://dist.mxnet.io/python
 else
   python3 -m pip uninstall --quiet mxnet-cu102 -y
   python3 -m pip install -U --quiet --pre "mxnet-cu102>=2.0.0a" --user