ci: Increase CPU number for "Win64 native" task

Currently, the time it takes to get the "Win64 native" task done with
all of the caches been invalidated is very close to the 2 hours limit.

This task is the only one which runs on Windows Community Cluster,
therefore this change should not affect other CI tasks.

.cirrus.yml

@@ -97,7 +97,7 @@ task:
   name: "Win64 native [vs2022]"
   << : *FILTER_TEMPLATE
   windows_container:
-    cpu: 4
+    cpu: 6
     memory: 8G
     image: cirrusci/windowsservercore:visualstudio2022
   timeout_in: 120m