Test pre-release notebooks after pre-release in isolation

[balopat]

Description of the issue

Our notebook test infra is pretty good but has one gap: pre-release notebooks are not tested in isolation, only on PRs against the given branch. This gap should be covered by a test that runs after a master commit triggered the pre-release to pypi, the new release is in the index and should cover all the notebooks that have pre-release cirq installed in them.

[95-martin-orion]

Do we know if this was already done? Otherwise it seems simple enough to add, but not priority for 1.0.