Switch FreeBSD's CI image to stable/11

The 11.3-release GCE image isn't working due to a bug in the included
py36-google-compute-engine package, but that bug is fixed in the latest
stable/11 images.

.cirrus.yml

 task:
   name: stable x86_64-unknown-freebsd-11
   freebsd_instance:
-    image: freebsd-11-3-release-amd64
+    image: freebsd-11-3-stable-amd64-v20190801
   setup_script:
     - pkg install -y curl
     - curl https://sh.rustup.rs -sSf --output rustup.sh