improve output for bbl up/destroy tasks

[will-gant]

What is this change about?

Fixes a small issue in the bbl-up task which causes it to fail to print the nameservers for the created DNS zone when deploying on AWS.

Please provide contextual information.

For some odd reason, bbl gives this output different names for aws and gcp. For the latter it's named system_domain_dns_servers, and for the former it's env_dns_zone_name_servers. As a result, this is the build output you currently get if you're running on aws (and presumably any other IaaS other than gcp):

Please configure DNS with the following servers:
+ bbl outputs
+ yq .system_domain_dns_servers -y
+ cut '-d ' -f2
null

Please check all that apply for this PR:

• [ ] introduces a new task
• [x] changes an existing task
• [ ] changes the Dockerfile
• [ ] introduces a breaking change (other users will need to make manual changes when this is released)

Did you update the README as appropriate for this change?

• [ ] YES
• [x] N/A

How should this change be described in release notes?

Bug fix

What is the level of urgency for publishing this change?

• [ ] Urgent - unblocks current or future work
• [ ] Slightly Less than Urgent