chore: upgrade CI runtime to Ubuntu 18.04 (#5327)
chore: upgrade CI runtime to Ubuntu 18.04 (#5327)

As GitHub issue indicates, runtime 16.04 is deprecated in favor of
ubuntu-latest or ubuntu-18.04. This change upgrades us to a pinned
version, 18.04.
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard