AMI build can fail due to locked apt/deb proc

the agent setup script is typically rubbish and im guessing using apt over apt-get - either way we can get errors like ...

```console
==> envoy-azp-build-arm64.amazon-ebs.envoy-azp-build-arm64: debconf: DbDriver "config": /var/cache/debconf/config.dat is locked by another process: Resource temporarily unavailable
```
