backend: add trailing slash if needed

Otherwise we can redirect to something like this:

https://console.redhat.com/api/pulp-content/public-copr-stage@copr/TEST1742912352410340284-storage-pulp/fedora-41-x86_64/

roles/copr/backend/templates/lighttpd/pulp-redirect.lua.j2

@@ -52,7 +52,14 @@ function pulp_url(copr_path)
   if string.sub(copr_path, 1, 9) == "/results/" then
     copr_path = string.sub(copr_path, 10)
   end
-  return pulp_content_url .. copr_path
+
+  local url = pulp_content_url
+  if string.sub(url, -1) ~= "/" then
+    url = url .. "/"
+  end
+
+  url = url .. copr_path
+  return url
 end