app: os.setenv() affects os.environ()

os.setenv() and os.environ() are Lua API for

    extern char **environ;
    int setenv();

The Open Group standardized access points for environment
variables. But there is no a word about that environ never
changes. Programs can't relay on that. For example, addition of
a new variable may cause realloc of the whole environ array, and
therefore change of its pointer value. That was exactly the case
in os.environ() - it was using value of environ array remembered
when Tarantool started.

And os.setenv() could realloc the array and turn the saved pointer
into garbage.

Closes #4733