As there will be releases 1.7.1, 1.7.2 etc., storing zsync in release
asset doesn't make sense (further version won't update previous update
information). Thus store this in continuous release assets - update
channel "release" was introduced aside to existing "continuous".
The release channel is not build from release/ branch, instead a bugfix
version numbers are required and the build is from appropriate tag, eg.
v1.7 (release, should be in master) and then v1.7.1, v1.7.2. The bugfix
releases should have its own entry in NEWS file (create-release.sh makes
GitHub release description from this).
