only install ubuntu-archive-keyring in buster or sid

manifests/init.pp

@@ -96,7 +96,15 @@ class su_debuild (
   ## Install ubuntu-archive-keyring
 
   # We add the Ubuntu keyring in case we want to build Ubuntu packages.
-  package {'ubuntu-archive-keyring': ensure => installed }
+  # This package is not (yet) in bullseye, so we only install it for
+  # buster, stretch, or sid.
+  case $::lsbdistcodename {
+    'buster', 'stretch', 'sid': {
+      package {
+         'ubuntu-archive-keyring': ensure => installed;
+      }
+    }
+  }
 
   # Additional packages to build kernel modules.
   case $::lsbdistcodename {