pgsql: Force the TZ environment variable to be set during initdb.

Log Message:
-----------
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.

Modified Files:
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    pgsql/src/bin/initdb:
        initdb.c (r1.178 -> r1.179)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/initdb/initdb.c?r1=1.178&r2=1.179)