Update msan job to Ubuntu 20.04

Also don't explicitly specify symbolizer path. This doesn't appear
to be necessary anymore, and avoids a dependence on the specific
LLVM version used.

azure/msan_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/test.yml

@@ -12,7 +12,6 @@ steps:
       export TEST_PHP_JUNIT=junit.xml
       export REPORT_EXIT_STATUS=no
       export SKIP_IO_CAPTURE_TESTS=1
-      export MSAN_SYMBOLIZER_PATH=/usr/lib/llvm-6.0/bin/llvm-symbolizer
       rm -rf junit.xml | true
       php run-tests.php -P -q \
           -j$(/usr/bin/nproc) \