VyOS builds now use the deb.debian.net load balanced mirror

If there are any good things about that packages server migration and restructuring is that it promoted a revamp of the associated part of the build scripts.

Since the start the default Debian mirror was set to nl.debian.org for a completely arbitrary reason. This of course was suboptimal for most users who are far from the Netherlands, and while the mirror is easy enough to change in ./configure options, a better out of the box experience wouldn't harm.

Danny ter Haar (fromport) suggested that we change it to deb.debian.org which is load balanced, which I think is a good idea. There's a small chance that it will redirect you to a dead mirror, but if you run into any issues, you can always set it by hand.