It is not a portable expectation that a single-shot environment
variable assignment works when calling a shell function, not a
command.

Set and export the variable before calling "test_must_fail git push"
instead.  This change would not hurt because this is the last
command in the subprocess and the environment will not seep through
to later tests without using a single-shot assignment.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>