glib's g_getenv() doc says: "The returned string may be overwritten by
the next call to g_getenv(), g_setenv() or g_unsetenv()". And I do
g_setenv() calls just after, while I wish to keep the value intact.

As a consequence, even though the previous commit seemed to work just fine,
I duplicate the return value of g_getenv(), just to be on the safe side.