dovecot: fix storage basedir regression in ldap home

During the rewrite of the LDAP userdb field lookups the default path for
the mail storage directory accidentally leaked into the home directory
path.

mail-server/dovecot.nix

@@ -79,7 +79,7 @@ let
       scope = ${mkLdapSearchScope cfg.ldap.scope}
       user_attrs = \
         ${ldapUuidAttribute}=${ldapUuidAttribute}, \
-        =home=/var/vmail/ldap/%{ldap:${ldapUuidAttribute}}, \
+        =home=${cfg.mailDirectory}/ldap/%{ldap:${ldapUuidAttribute}}, \
         =mail=maildir:~/mail${maildirLayoutAppendix}${maildirUTF8FolderNames}${
           lib.optionalString (cfg.indexDir != null) ":INDEX=${cfg.indexDir}/ldap/%{ldap:${ldapUuidAttribute}}"
         }