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>