Use index-url for torchtext installs (#2075)

Author: atalman

.github/scripts/validate_binaries.sh

@@ -7,7 +7,7 @@ if [[ ${MATRIX_PACKAGE_TYPE} = "conda" ]]; then
         conda install -y torchtext -c ${PYTORCH_CONDA_CHANNEL}
     fi
 else
-    pip install ${PYTORCH_PIP_PREFIX} torchtext --extra-index-url ${PYTORCH_PIP_DOWNLOAD_URL}
+    pip install ${PYTORCH_PIP_PREFIX} torchtext --index-url ${PYTORCH_PIP_DOWNLOAD_URL}
 fi
 
 python  ./test/smoke_tests/smoke_tests.py