CI: use "GIT_TEST_SANITIZE_LEAK_LOG=true" in linux-leaks

CI: use "GIT_TEST_SANITIZE_LEAK_LOG=true" in linux-leaks

As noted in a preceding commit the leak checking done by
"GIT_TEST_PASSING_SANITIZE_LEAK=true" (added in [1]) is incomplete
without combining it with "GIT_TEST_SANITIZE_LEAK_LOG=true".

Let's run our CI with that, to ensure that we catch cases where our
tests are missing the abort() exit code resulting from a leak for
whatever reason. The reasons for that are discussed in detail in a
preceding commit.

1. 956d2e4639b (tests: add a test mode for SANITIZE=leak, run it in
   CI, 2021-09-23)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>