fix(cli): cross-account asset publishing doesn't work without bootstrap stack

[rix0rrr]

When determining whether or not to allow cross-account bucket uploads, we inspect the bootstrap stack in the account we're deploying to:

• If it doesn't advertise an S3 Bucket, then we allow cross-account bucket uploads (if the bucket isn't in the bootstrap stack in our account, it's probably in a different account, because we do need one)
• if the bootstrap stack is recent enough, we also allow cross-account bucket uploads (there are mitigations against problems in the IAM policy of recent bootstrap stacks, and if those are gone they have been removed on purpose by the customer).

However, if the bootstrap stack can't be found, it can't be validated. This can happen if the customer provisions their resources differently than via the bootstrap stack.

When that happens, we used to fail closed: but that just means that cross-account asset uploads are now prohibited, and not having a bootstrap stack and expecting assets to be uploaded to a different account in your organization should be a valid setup.

So instead, we have to fail open: if we can't find the bootstrap stack, we do allow cross-account asset uploads. We treat this check more as a best-effort sanity check. This is not the only protection mechanism we have, so the local check is more of a bonus.

Fixes #31866.

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