Update ubuntu on i386 job

There is a failure with apt on PHP-7.4 that's not present on
PHP-8.0/master. Switching to a newer ubuntu version on PHP-7.4
avoids the issue.

azure/i386/job.yml

@@ -5,7 +5,7 @@ parameters:
 jobs:
   - job: ${{ parameters.configurationName }}
     pool:
-      vmImage: 'ubuntu-18.04'
+      vmImage: 'ubuntu-20.04'
     steps:
     - template: apt.yml
     - script: |