core.cfg: make detection of sysconfdir version independent
core.cfg: make detection of sysconfdir version independent

When LuaRocks is running on Lua 5.3 and a user uses --lua-version=5.1,
this allows sysconfdir to continue being detected correctly.

tarantool/luarocks,keplerproject/luarocks,luarocks/luarocks,tarantool/luarocks,keplerproject/luarocks,keplerproject/luarocks,keplerproject/luarocks,luarocks/luarocks,tarantool/luarocks,luarocks/luarocks