ci: Increase requested memory size.

CI builds recently started failing with:

"Memory size for 4.0 vCPU instance should be between 3840MiB and
26624MiB, while 2048MiB is requested."

Ok then, let's ask for 4G instead of 2G.

This may be due to a change in the type of instance used to work around
an outage, per:

https://twitter.com/cirrus_labs/status/1572657320093712384

.cirrus.yml

@@ -55,7 +55,7 @@ task:
     image: family/pg-ci-freebsd-13
     platform: freebsd
     cpu: $CPUS
-    memory: 2G
+    memory: 4G
     disk: 50
 
   sysinfo_script: |
@@ -166,7 +166,7 @@ task:
     image: family/pg-ci-bullseye
     platform: linux
     cpu: $CPUS
-    memory: 2G
+    memory: 4G
 
   ccache_cache:
     folder: ${CCACHE_DIR}