t5608-clone-2gb.sh: turn GIT_TEST_CLONE_2GB into a bool

t5608-clone-2gb.sh: turn GIT_TEST_CLONE_2GB into a bool

The GIT_TEST_CLONE_2GB environment variable is only ever checked with
'test -z' in 't5608-clone-2gb.sh', so any non-empty value is
interpreted as "yes, run these expensive tests", even
'GIT_TEST_CLONE_2GB=NoThanks'.

Similar GIT_TEST_* environment variables have already been turned into
bools in 3b072c577b (tests: replace test_tristate with "git
env--helper", 2019-06-21), so let's turn GIT_TEST_CLONE_2GB into a
bool as well, to follow suit.

Our CI builds set GIT_TEST_CLONE_2GB=YesPlease, so adjust them
accordingly, thus removing the last 'YesPlease' from our CI scripts.

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