Use sieve_default option for sieveScript

https://wiki2.dovecot.org/Pigeonhole/Sieve/Configuration#Visible_Default_Script

mail-server/dovecot.nix

@@ -110,6 +110,8 @@ in
 
         plugin {
           sieve = file:/var/sieve/%u/scripts;active=/var/sieve/%u/active.sieve
+          sieve_default = file:/var/sieve/%u/default.sieve
+          sieve_default_name = default
         }
 
         lda_mailbox_autosubscribe = yes

mail-server/users.nix

@@ -55,16 +55,21 @@ let
     # is null, remove the file.
     ${lib.concatMapStringsSep "\n" ({ name, sieveScript }:
       if lib.isString sieveScript then ''
-        cat << EOF > "/var/sieve/${name}.sieve"
+        if (! test -d "/var/sieve/${name}"); then
+          mkdir -p "/var/sieve/${name}"
+          chown "${name}:${vmailGroupName}" "/var/sieve/${name}"
+          chmod 770 "/var/sieve/${name}"
+        fi
+        cat << EOF > "/var/sieve/${name}/default.sieve"
         ${sieveScript}
         EOF
-        chown "${name}:${vmailGroupName}" "/var/sieve/${name}.sieve"
+        chown "${name}:${vmailGroupName}" "/var/sieve/${name}/default.sieve"
       '' else ''
-        if (test -f "/var/sieve/${name}.sieve"); then
-          rm "/var/sieve/${name}.sieve"
+        if (test -f "/var/sieve/${name}/default.sieve"); then
+          rm "/var/sieve/${name}/default.sieve"
         fi
         if (test -f "/var/sieve/${name}.svbin"); then
-          rm "/var/sieve/${name}.svbin"
+          rm "/var/sieve/${name}/default.svbin"
         fi
       '') (map (user: { inherit (user) name sieveScript; })
             (lib.attrValues loginAccounts))}