Rewrite debian mirror url based on the CC mirror selected for Devuan.
It seems that for some countries, debian httpredir doesn't work very well. For example here in New Zealand ftp.nz.debian.org is my local mirror and only a couple of hops away, but debians httpredir mirror uses either australian or us based mirrors.
I'm proposing that we use the CC derived from CC.mirror.devuan.org as a selector also for the debian mirror. This will in many cases significantly reduce the time it takes for installation/updates of devuan.
initially we could trial it by using the CC "nz" to use ftp.nz.debian.org