ci/cirrus: limit numcpu

... so we can run all four jobs in parallel.

While at it, fix the comment in the file.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.cirrus.yml

@@ -1,7 +1,8 @@
 ---
-# We use Cirrus for Vagrant tests and native CentOS 7 and 8, because macOS
-# instances of GHA are too slow and flaky, and Linux instances of GHA do not
-# support KVM.
+# We use Cirrus for CentOS (native) and Fedora (in Vagrant), because neither
+# CentOS nor Fedora is available on GHA natively, so the only option is VM.
+# In GHA, nested virtualization is only supported on macOS instances, which
+# are slow and flaky.
 
 # NOTE Cirrus execution environments lack a terminal, needed for
 # some integration tests. So we use `ssh -tt` command to fake a terminal.
@@ -24,9 +25,9 @@ task:
     platform: linux
     nested_virtualization: true
     # CPU limit: `16 / NTASK`: see https://cirrus-ci.org/faq/#are-there-any-limits
-    cpu: 8
+    cpu: 4
     # Memory limit: `4GB * NCPU`
-    memory: 32G
+    memory: 16G
 
   host_info_script: |
     uname -a