Related: #1979490 Need a default for .el8 without _N (5 for now)

(cherry picked from commit cbd5f6c2a4de0c8033c31e447a5dc4a00c24a0cb)

thunderbird.spec

@@ -16,6 +16,10 @@ function dist_to_rhel_minor(str, start)
   if match then
      return string.sub(match, 6)
   end
+  match = string.match(str, ".el8")
+  if match then
+     return 5
+  end
   return -1
 end}