Add check for S3 bucket existence prior to Cypress tests in build-and-deploy.yml

[thoward]

The workflow https://github.com/pulumi/docs/blob/master/.github/workflows/build-and-deploy.yml frequently fails due to Cypress getting a timeout when trying to retrieve the site. This is likely due to a bucket creation race condition where the bucket is not yet retreivable but also doesn't error, leading to a timeout when trying to fetch it. We can make this job more resilient to this type of problem by adding a wait/check loop prior to the Cypress tests, which will validate and fetch the S3 bucket contents before Cypress runs. This should have reasonable timeouts/retry limits to prevent hangs.

See #15000 for context.