Add an older Alpine CI build on Cirrus

This is the oldest one still supported and ensures that the code still
compiles there. Also, reduce the resource requirements a bit. One CPU is
plenty because there is very little parallelization going on.

.cirrus.yml

@@ -4,6 +4,8 @@
 freebsd_task:
   name: FreeBSD
   freebsd_instance:
+    memory: 1500M
+    cpu: 1
     matrix:
       image_family: freebsd-13-2
       image_family: freebsd-12-4
@@ -46,6 +48,7 @@ linux_arm_task:
   arm_container:
     image: ubuntu:22.04
     memory: 400M
+    cpu: 1
 
   env:
     # Set to 10 instead of 1 to avoid problems when new commits are submitted
@@ -84,8 +87,11 @@ linux_arm_task:
 linux_musl_task:
   name: Linux MUSL
   container:
-    image: alpine:3.18
-    memory: 600M
+    memory: 400M
+    cpu: 1
+    matrix:
+      image: alpine:3.18
+      image: alpine:3.16
 
   env:
     # Set to 10 instead of 1 to avoid problems when new commits are submitted