t0090: disable GIT_TEST_SPLIT_INDEX for the test checking split index

t0090: disable GIT_TEST_SPLIT_INDEX for the test checking split index

The test 'switching trees does not invalidate shared index' in
't0090-cache-tree.sh' is about verifying the behaviour of the split
index feature, therefore it should be in full control of when index
splitting is performed, like all the tests in 't1700-split-index.sh'.

Unset GIT_TEST_SPLIT_INDEX for this test to avoid unintended random
index splitting.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>