Merge pull request #1664 from JohnTitor/catalina

Switch macOS images to 10.14

Author: JohnTitor

ci/azure.yml

@@ -189,7 +189,7 @@ jobs:
   - job: BuildChannelsOSX
     dependsOn: StyleAndDocs
     pool:
-      vmImage: macos-10.13
+      vmImage: macos-10.14
     steps:
       - template: azure-install-rust.yml
       - script: LIBC_CI=1 sh ./ci/build.sh