dovecot: make sure vid/gid are not overridable

The only storage scheme we support is a single declarative user with
fixed uid/gid. The default_fields are overridable if these fields leak
in from LDAP, so promote them to override_fields instead.

mail-server/dovecot.nix

@@ -445,7 +445,7 @@ in
           userdb {
             driver = ldap
             args = ${ldapConfFile}
-            default_fields = \
+            override_fields = \
               uid=${toString cfg.vmailUID} \
               gid=${toString cfg.vmailUID}
           }