ci: Use VMs for SanityCheck and CompilerWarnings

The main reason for this change is to reduce different ways of executing
tasks, making it easier to use custom compute resources for cfbot. A secondary
benefit is that the tasks seem slightly faster this way, apparently the
increased startup overhead is outweighed by reduced runtime overhead.

Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/20230808021541.7lbzdefvma7qmn3w@awork3.anarazel.de
Backpatch: 15-, where CI support was added

.cirrus.yml

@@ -498,6 +498,7 @@ task:
   env:
     CPUS: 4
     BUILD_JOBS: 4
+    IMAGE_FAMILY: pg-ci-bullseye
 
     # Use larger ccache cache, as this task compiles with multiple compilers /
     # flag combinations
@@ -510,9 +511,7 @@ task:
   # condition here ...
   only_if: $CIRRUS_CHANGE_MESSAGE !=~ '.*\nci-os-only:.*' || $CIRRUS_CHANGE_MESSAGE =~ '.*\nci-os-only:[^\n]*linux.*'
 
-  container:
-    image: $CONTAINER_REPO/linux_debian_bullseye_ci:latest
-    cpu: $CPUS
+  <<: *linux_task_template
 
   sysinfo_script: |
     id