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(thanks to Vyacheslav for troubleshooting). It looks like TOML files under data/build-flavors should have some instructions about additional repositories.