generate-svn-snapshot: use UNRELEASED for snapshots builds
generate-svn-snapshot: use UNRELEASED for snapshots builds

We do the same in generate-git-snapshots, so let's make sure unreleased
builds use UNRELEASED instead of unstable (which is useful for released
packages).

Thanks: Christian Schneider <2314b2e3a4a1f7db165be2aafbf1efd78f28cc97@chrisn.dtdns.net> for the patch

kartikm/jenkins-debian-glue,cclausen/jenkins-debian-glue,n054/jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,n054/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,mika/jenkins-debian-glue,webtrekk-kane/jenkins-debian-glue,kartikm/jenkins-debian-glue,cclausen/jenkins-debian-glue,mika/jenkins-debian-glue,linuxmaniac/jenkins-debian-glue,bmiklautz/jenkins-debian-glue,jbfavre/jenkins-debian-glue,bmiklautz/jenkins-debian-glue,hashar/jenkins-debian-glue,tonin/jenkins-debian-glue,jbfavre/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,hetznerZA/jenkins-debian-glue,hetznerZA/jenkins-debian-glue,tonin/jenkins-debian-glue,wikimedia/operations-debs-jenkins-debian-glue,hashar/jenkins-debian-glue