ci: increase CPU count of sanitizer job to increase memory limit

According to the docs,
https://cirrus-ci.org/guide/linux/#linux-containers, "For each CPU you
can't get more than 4G of memory.", thus if we want this job to have
24GB of memory, we need to increase the CPU count to 6.

.cirrus.yml

@@ -113,7 +113,7 @@ task:
   << : *GLOBAL_TASK_TEMPLATE
   container:
     image: ubuntu:focal
-    cpu: 4  # Double CPU and increase Memory to avoid timeout
+    cpu: 6  # Increase CPU and Memory to avoid timeout
     memory: 24G
   env:
     MAKEJOBS: "-j8"