ci: pin tf-nightly to 20210614 (#5068)
ci: pin tf-nightly to 20210614 (#5068)

With the latest nightly (2.6.0.dev20210616), TensorBoard CI fails because hparams
API fails to load properly when importing TensorFlow inside TensorBoard
project.

Specifically, below fails.

```sh
# cd into TensorBoard repository
cd tensorboard
python
>>> import tensorflow as tf
```

This may have to do with summaries are exported under TensorFlow and
requires a bit more precise diagnosis. Until then, we will pin the
tf-nightly to the last good version (2.6.0.dev20210614).

Suspected breaking change: https://github.com/tensorflow/tensorflow/commit/2e6acf753381cb7902017bcab03e204bb41b7091
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard