Update community, coverage and file cache jobs to Ubuntu 20.04

This fixes the issue with apt, and seems like a good idea in
general. The msan job is now the only one on 18.04, but that one
needs more work.

Author: nikic

azure/community_job.yml

@@ -16,7 +16,7 @@ jobs:
   - job: ${{ parameters.configurationName }}
     timeoutInMinutes: ${{ parameters.timeoutInMinutes }}
     pool:
-      vmImage: 'ubuntu-18.04'
+      vmImage: 'ubuntu-20.04'
     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-18.04'
+      vmImage: 'ubuntu-20.04'
     steps:
     - template: apt.yml
     - script: |

azure/file_cache_job.yml

@@ -8,7 +8,7 @@ jobs:
   - job: ${{ parameters.configurationName }}
     timeoutInMinutes: ${{ parameters.timeoutInMinutes }}
     pool:
-      vmImage: 'ubuntu-18.04'
+      vmImage: 'ubuntu-20.04'
     steps:
     - template: apt.yml
     - template: configure.yml