Attempt to fix CI test failures on library mapgen

[Maleclypse]

Summary

None

Purpose of change

Bugfix test failures.

(continued from above) ERROR : src/map.cpp:1662 [bool map::furn_set(const tripoint &, const furn_id &, const bool, bool)] Setting furniture f_water_heater at (16,17,1) where terrain is t_open_air (which is_open_air)
([slow] ~starting_items)=> If this is intentional, set the ALLOW_ON_OPEN_AIR flag on the furniture
([slow] ~starting_items)=> 01:22:08.477 ERROR : src/mapgen.cpp:2766 [virtual void jmapgen_furniture::apply(const mapgendata &, const jmapgen_int &, const jmapgen_int &, const std::string &) const] Problem setting furniture in nested mapgen roof_6x6_utility in mapgen s_library_roof_3
([slow] ~starting_items)=> 01:22:08.477 ERROR : src/map.cpp:1662 [bool map::furn_set(const tripoint &, const furn_id &, const bool, bool)] Setting furniture f_water_heater at (17,17,1) where terrain is t_open_air (which is_open_air)
([slow] ~starting_items)=> If this is intentional, set the ALLOW_ON_OPEN_AIR flag on the furniture
([slow] ~starting_items)=> 01:22:08.477 ERROR : src/mapgen.cpp:2766 [virtual void jmapgen_furniture::apply(const mapgendata &, const jmapgen_int &, const jmapgen_int &, const std::string &) const] Problem setting furniture in nested mapgen roof_6x6_utility in mapgen s_library_roof_3
([slow] ~starting_items)=> 01:22:08.477 ERROR : src/map.cpp:1662 [bool map::furn_set(const tripoint &, const furn_id &, const bool, bool)] Setting furniture f_water_purifier at (18,17,1) where terrain is t_open_air (which is_open_air)
([slow] ~starting_items)=> If this is intentional, set the ALLOW_ON_OPEN_AIR flag on the furniture
([slow] ~starting_items)=> 01:22:08.477 ERROR : src/mapgen.cpp:2766 [virtual void jmapgen_furniture::apply(const mapgendata &, const jmapgen_int &, const jmapgen_int &, const std::string &) const] Problem setting furniture in nested mapgen roof_6x6_utility in mapgen s_library_roof_3217.484 s: overmap_terrain_coverage

Describe the solution

Trying to fix the above error.

Describe alternatives you've considered

leaving for others

Testing

CI and hope it's better

Additional context