Keep dots in branch name when converting to APT suite name.

That's how the conversion is done on the APT repository side.

auto/scripts/tails-custom-apt-sources

@@ -39,7 +39,7 @@ on_topic_branch() {
 branch_name_to_suite() {
 	local branch="$1"
 
-	echo "$branch" | sed -e 's,[^a-z0-9-],-,ig'  | tr '[A-Z]' '[a-z]'
+	echo "$branch" | sed -e 's,[^.a-z0-9-],-,ig'  | tr '[A-Z]' '[a-z]'
 }