Update job images to 18.04

Author: JohnTitor

ci/azure.yml

@@ -12,7 +12,7 @@ pr: ["master"]
 jobs:
   - job: DockerLinuxTier1
     pool:
-      vmImage: ubuntu-16.04
+      vmImage: ubuntu-18.04
     steps:
       - template: azure-install-rust.yml
       - bash: LIBC_CI=1 sh ./ci/run-docker.sh $TARGET
@@ -27,7 +27,7 @@ jobs:
   - job: DockerLinuxTier2
     #dependsOn: DockerLinuxTier1
     pool:
-      vmImage: ubuntu-16.04
+      vmImage: ubuntu-18.04
     steps:
       - template: azure-install-rust.yml
       - bash: LIBC_CI=1 sh ./ci/run-docker.sh $TARGET