[devops] Look in the correct directory for the API diff results. (xam…

…arin#20624)

Fixes this failure:

> 🔥 Unable to find the contents for the comment: D:\a\1\a\change-detection\results\gh-comment.md does not exist :fire

This is a partial revert of xamarin#20601, which broke the API diff.

tools/devops/automation/templates/build/api-diff-process-results.yml

@@ -69,7 +69,7 @@ steps:
       Import-Module $Env:SYSTEM_DEFAULTWORKINGDIRECTORY\xamarin-macios\tools\devops\automation\scripts\MaciosCI.psd1
       $vsdropsChangeDetectionPrefix = "https://vsdrop.corp.microsoft.com/file/v1/xamarin-macios/detected-changes/$Env:BUILD_BUILDNUMBER/$Env:BUILD_BUILDID-$Env:SYSTEM_JOBATTEMPT/;/"
 
-      $rootDirectory = Join-Path "$Env:BUILD_ARTIFACTSTAGINGDIRECTORY" "change-detection" "results"
+      $rootDirectory = Join-Path "$Env:SYSTEM_DEFAULTWORKINGDIRECTORY" "change-detection" "results"
 
       $inputContentsPath = Join-Path -Path $rootDirectory -ChildPath "gh-comment.md"
       if (Test-Path $inputContentsPath -PathType leaf) {