Put Release file in stable/nightly folders of APT repo

[HebaruSan]

Problem

The APT repo from #3197 doesn't work yet.

Fehl:22 https://ksp-ckan.s3-us-west-2.amazonaws.com/deb nightly Release
404  Not Found [IP: 52.218.233.33 443]
Paketlisten werden gelesen... Fertig
E: Das Depot »https://ksp-ckan.s3-us-west-2.amazonaws.com/deb nightly Release« enthält keine Release-Datei

Cause

We're supposed to have a file at /deb/dists/nightly/Release:

s3-us-west-2-r-w.amazonaws.com	http	HTTP		250		GET /deb/dists/nightly/InRelease HTTP/1.1 
s3-us-west-2-r-w.amazonaws.com	http	HTTP		248		GET /deb/dists/nightly/Release HTTP/1.1 

Mono has this.

• https://download.mono-project.com/repo/ubuntu/dists/stable-bionic/

Changes

Now we copy the Release file into an additional folder at _build/deb/apt-repo-rel, and sync that folder to deb/dists/nightly and/or deb/dists/stable.

[DasSkelett]

Makefiles are so cryptic.
Let's give it a try.