[CI] Fix mac tests on post build pipeline (#20543)

To be able to download a artifact from a build that triggered the post
build pipeline we cannot use the DownloadPipelineArtifact task with the
same values since the artifact wont be found. This change adds an if
statement to do the right thing based on the trigger so that we can have
both pipelines running correctly until we remove the tests from the
build.

---------

Co-authored-by: Rolf Bjarne Kvinge <rolf@xamarin.com>

tools/devops/automation/templates/mac/build.yml

@@ -137,13 +137,30 @@ steps:
   displayName: 'Remove Provisionator Tool Cache'
   condition: ${{ parameters.clearProvisionatorCache }}
 
-# Go get the tests!
-- task: DownloadPipelineArtifact@2
-  displayName: Download Mac tests
-  inputs:
+# Go get the tests!, this depends on how the test was triggered
+- ${{ if or(contains(variables['Build.Reason'], 'ResourceTrigger'), contains(variables['Build.Reason'], 'BuildCompletion')) }}:
+  - download: macios
+    artifact: mac-test-package
+    displayName: Download Mac tests
     patterns: '**/mac-test-package.7z'
-    allowFailedBuilds: true
-    path: $(Build.SourcesDirectory)/artifacts/tmp
+
+  # the default location when downloading is $(Pipeline.Workspace)/<pipeline resource identifier>/<artifact name>
+  - pwsh: |
+      $source = "$(Pipeline.Workspace)/macios/mac-test-package"
+      $destination = "$(Build.SourcesDirectory)/artifacts/tmp/mac-test-package"
+      New-Item -ItemType Directory -Force -Path $destination 
+      Write-Host "Moving content from $source to $destination"
+      # move all the files from the source to the destination
+      Get-ChildItem -Path $source -Recurse -File | Move-Item -Destination $destination 
+    displayName: Move artifacts to the expected location
+
+- ${{ else }}:
+  - task: DownloadPipelineArtifact@2
+    displayName: Download Mac tests
+    inputs:
+      patterns: '**/mac-test-package.7z'
+      allowFailedBuilds: true
+      path: $(Build.SourcesDirectory)/artifacts/tmp
 
 - bash: |
     set -ex