[Docker] Install oneflow from PyPi (#15819)

Installing oneflow from the current link (https://release.oneflow.info)
seems to be broken as reported in #15754, which is impacting other
unrelated changes in CI. This commit attempts to fix the install by
using a version from PyPi.

docker/install/ubuntu_install_oneflow.sh

@@ -22,4 +22,4 @@ set -o pipefail
 
 pip3 install flowvision==0.1.0
 
-python3 -m pip install -f https://release.oneflow.info oneflow==0.7.0+cpu
+python3 -m pip install oneflow==0.7.0