Explicitly specify ubuntu versions on azure pipelines

Use the new 18.04 for the ones where the build succeeded and the
previous 16.04 where it didn't.

Author: nikic

azure/community_job.yml

@@ -16,7 +16,7 @@ jobs:
   - job: ${{ parameters.configurationName }}
     timeoutInMinutes: ${{ parameters.timeoutInMinutes }}
     pool:
-      vmImage: 'ubuntu-latest'
+      vmImage: 'ubuntu-16.04'
     variables:
       ubsan_options: 'print_stacktrace=1'
     steps:

azure/coverage_job.yml

@@ -8,7 +8,7 @@ jobs:
   - job: ${{ parameters.configurationName }}
     timeoutInMinutes: ${{ parameters.timeoutInMinutes }}
     pool:
-      vmImage: 'ubuntu-latest'
+      vmImage: 'ubuntu-18.04'
     steps:
     - template: apt.yml
     - script: |

azure/i386/job.yml

@@ -5,7 +5,7 @@ parameters:
 jobs:
   - job: ${{ parameters.configurationName }}
     pool:
-      vmImage: 'ubuntu-latest'
+      vmImage: 'ubuntu-16.04'
     steps:
     - template: apt.yml
     - script: |

azure/job.yml

@@ -8,7 +8,7 @@ jobs:
   - job: ${{ parameters.configurationName }}
     timeoutInMinutes: ${{ parameters.timeoutInMinutes }}
     pool:
-      vmImage: 'ubuntu-latest'
+      vmImage: 'ubuntu-18.04'
     steps:
     - template: apt.yml
     - template: configure.yml

azure/msan_job.yml

@@ -8,7 +8,7 @@ jobs:
   - job: ${{ parameters.configurationName }}
     timeoutInMinutes: ${{ parameters.timeoutInMinutes }}
     pool:
-      vmImage: 'ubuntu-latest'
+      vmImage: 'ubuntu-18.04'
     steps:
     - template: apt.yml
     - script: |