Force the TZ environment variable to be set during initdb. This is to

short-circuit the rather expensive identify_system_timezone() procedure,
which we have no real need for during initdb since nothing done here depends
on the timezone setting.  Since we launch quite a few standalone backends
during the initdb sequence, this adds up to a significant savings, and seems
worth doing to save developer time even though it will hardly matter to end
users.  Per my report today on pgsql-hackers.