The Zabbix compatibility tests workflow were failing with Docker build
errors:
```
E: The repository 'http://deb.debian.org/debian buster Release' does not have a Release file.
E: The repository 'http://deb.debian.org/debian buster-updates Release' does not have a Release file.
E: The repository 'http://security.debian.org/debian-security buster/updates Release' does not have a Release file.
```
The `python:2.7` Docker image is based on Debian Buster, which reached
end-of-life in August 2022. The Debian repositories for Buster have been
moved from their original locations to `archive.debian.org`, but the
Docker image still points to the original URLs.
This PR updates `devenv/zas-agent/Dockerfile` to redirect repository
sources to use Debian's archive repositories:
- `http://deb.debian.org/debian` → `http://archive.debian.org/debian`
- `http://security.debian.org/debian-security` →
`http://archive.debian.org/debian-security`
This allows the Python 2.7 environment to continue working with the
zas_agent dependency, which requires Python 2.7 syntax.
