add @ to environ constraints

Signed-off-by: Mattias Andrée <maandree@operamail.com>
=VAR means the variable must not be defined if it is not defined for this process. Otherwise =VAR would be incorrectly interpreted as +=VAR if VAR is not defined here

Signed-off-by: Mattias Andrée <maandree@operamail.com>
info: environment constraints

Signed-off-by: Mattias Andrée <maandree@operamail.com>