ci: fix the linux-leaks job (git-for-windows#4085)

The CI builds have been seriously broken as of v2.38.1 by the constantly
failing `linux-leaks` job. Let's backport the work-around that has been
applied to upstream Git (but that funnily has not been based directly on
top of v2.38.1).

Author: dscho

Makefile

@@ -1341,6 +1341,7 @@ BASIC_CFLAGS += -DSHA1DC_FORCE_ALIGNED_ACCESS
 endif
 ifneq ($(filter leak,$(SANITIZERS)),)
 BASIC_CFLAGS += -DSUPPRESS_ANNOTATED_LEAKS
+BASIC_CFLAGS += -O0
 SANITIZE_LEAK = YesCompiledWithIt
 endif
 ifneq ($(filter address,$(SANITIZERS)),)