Fix issue of branch switching not working with bazel

This fix tries to address the issue raised in 15957 where
bazel stops working after switching git branch, and
reconfigure with `./configure` will not work as well.

This fix adds a quick fix as was suggested, by having
export TF_CONFIG_TIME="$(date)" in configure.py and add
it to the environ list in git_configure.bzl.

This fix fixes 15957.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>