fall back to alternative Zeek .deb download URL

[mmguero]

Found by and discussed with myself and @piercema

When Zeek posts new official debs, the URL for the old ones (we're using https://downloadcontentcdn.opensuse.org/repositories/security:/zeek/ ...) doesn't work any more. This means that it becomes impossible to build Zeek containers for older Malcolm versions.

In zeek-deb-download.sh, if the URL_PREFIX for whatever that deb gives a 404, we should fall back to another URL, if we can identify what that would be.

[mmguero]

So there isn't an official place where old Debs are kept, but I've added code that allows the build scripts to take a URL as a parameter (which can also be specified via an environment variable or GitHub repository secret) that will allow us to cache them internally should we choose to.