cli: pre-release integration test strategy for downstream tools (e.g. Amplify)

Today, any tools depending on the AWS CDK are forced to wait for a CDK release before they can execute their test suites against the latest version of the CDK. This is bad, because a change in the CDK might unintentionally break the other tool. Instead we should establish a pattern to test against pre-builds (nightlies) from main.

Primary concern will be for 1st party tools, however this could be extended to 2nd and 3rd party tooling as well.