Reduce the max number of jobs in Cirrus CI to 12

With 16 the build is still terminated by the OOM killer sometimes.

.cirrus.yml

@@ -28,8 +28,8 @@ task:
     wxPROC_COUNT=`./build/tools/proc_count.sh`
     # ARM systems have 64 CPUs but run out of memory and crash when using that
     # many jobs, so limit them to something reasonable.
-    if [ $wxPROC_COUNT -gt 16 ]; then
-      wxPROC_COUNT=16
+    if [ $wxPROC_COUNT -gt 12 ]; then
+      wxPROC_COUNT=12
     fi
 
     echo wxBUILD_ARGS=-j$wxPROC_COUNT >> $CIRRUS_ENV