I've tested all flavor targets for build-vyos-image in 1.5 and 1.4. In 1.5 they work fine. In 1.4 it looks like cloud-init couldn't initiate debian repositories appropriately and build process fails.
En example for aws-iso:
```
P: Begin installing packages (install pass)...
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
E: Unable to locate package amazon-cloudwatch-agent
```
UPD it fails in 1.5 too. The difference is that some packages were manually added to VyOS dev package repository. It looks like TOML files under data/build-flavors should have some instructions about additional repositories.