Use Ubuntu 16.04 to deploy GitHub Pages

Author: JohnTitor

ci/azure-master.yml

@@ -12,7 +12,8 @@ pr: ["master"]
 jobs:
   - job: StyleAndDocs
     pool:
-      vmImage: ubuntu-18.04
+      # FIXME: It seems Ubuntu 18.04 doesn't work well with simpleinfra.
+      vmImage: ubuntu-16.04
     steps:
       - template: azure-install-rust.yml
       - script: LIBC_CI=1 sh ci/dox.sh