[devops] Append to the agent log, instead of creating the file.

Append to the agent log, instead of creating the file, when an exception occcurs.

The problem is that creating the file fails if it already exists:

    Exception occurred: Exception calling "Parse" with "1" argument(s): "String '' was not recognized as a valid DateTime."
    New-Item: /Users/builder/azdo/_work/4/s/xamarin-macios/tools/devops/automation/scripts/generate_agent_logs.ps1:52
    Line |
      52 |      New-Item -Path $Output -Value "$_"
         |      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
         | The file '/Users/builder/azdo/_work/4/s/agent-logs.log' already exists.

tools/devops/automation/scripts/generate_agent_logs.ps1

@@ -46,8 +46,7 @@ try {
       log show --predicate $Predicate --style $Style --start "$start" --end "$end" > $Output
   }
 } catch {
-    Write-Host "Exception occurred: $_"
     # Create the output file, because we later try to upload it as an artifact, and *not* uploading
     # if there's *no* file is much harder than just creating the file.
-    New-Item -Path $Output -Value "$_"
+    Write-Host "Exception occurred: $_" | Tee-Object -FilePath $Output -Append
 }