Try to install/test arch completion only for the mailman one, not coreutils/util-linux-ng.

contrib/mailman

@@ -378,7 +378,9 @@ _config_list()
 } &&
 complete -F _config_list -o filenames config_list
 
-have arch &&
+# Try to detect whether this is the mailman "arch" to avoid installing
+# it for the coreutils/util-linux-ng one.
+have arch && have mailmanctl &&
 _arch()
 {
     local cur prev split=false

test/completion/arch.exp

@@ -1,3 +1,3 @@
-if {[assert_bash_type {arch}]} {
+if {[assert_bash_type {arch}] && [assert_bash_type {mailmanctl}]} {
     source "lib/completions/arch.exp"
 }; # if