t5801: "VAR=VAL shell_func args" is forbidden

t5801: "VAR=VAL shell_func args" is forbidden

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: Junio C Hamano <gitster@pobox.com>