Run Unpackaged Unit Tests with winget-installed PsExec (#4760)

Currently forked builds do not have the permission to access secure
files.

To workaround using PsExec from a secure file, we will use a stable
release of winget to install the latest PsTools and try invoking unit
tests from the PsExec executable that is installed as part of the zip
package.

azure-pipelines.yml

@@ -348,18 +348,30 @@ jobs:
     inputs:
       packageFeedSelector: 'nugetOrg'
 
-  - task: DownloadSecureFile@1
-    name: PsExec
-    displayName: 'Download PsExec.exe'
-    inputs:
-      secureFile: 'PsExec.exe'
+  # TODO: Repair-WinGetPackageManager will fail because it tries to install x64 for an x86 build machine.
+  # Remove manual installation of VCLibs (x64) once this is fixed.
+  - powershell: |
+      if ("$(buildPlatform)" -eq "x86") {
+        iwr https://aka.ms/Microsoft.VCLibs.x64.14.00.Desktop.appx -OutFile Microsoft.VCLibs.x64.appx
+        Add-AppxPackage Microsoft.VCLibs.x64.appx
+      }
+      Install-PackageProvider -Name NuGet -MinimumVersion 2.8.5.201 -Force
+      Install-Module Microsoft.WinGet.Client -Repository PSGallery -Force
+      Repair-WingetPackageManager -AllUsers -Latest
+      Install-WinGetPackage -Id Microsoft.Sysinternals.PsTools -Source winget
+    displayName: Install Sysinternals PsTools Using Winget
+    condition: succeededOrFailed()
 
-  - task: CmdLine@2
+  - powershell: |
+      $env:Path = [System.Environment]::GetEnvironmentVariable("Path","Machine") + ";" + [System.Environment]::GetEnvironmentVariable("Path","User")
+      PsExec -accepteula -s -i $(buildOutDir)\AppInstallerCLITests\AppInstallerCLITests.exe -logto $(artifactsDir)\AICLI-Unpackaged-System.log -s -r junit -o $(artifactsDir)\TEST-AppInstallerCLI-Unpackaged-System.xml
     displayName: Run Unit Tests Unpackaged Under System Context
-    inputs:
-      script: |
-        $(PsExec.secureFilePath) -accepteula -s -i $(buildOutDir)\AppInstallerCLITests\AppInstallerCLITests.exe -logto $(artifactsDir)\AICLI-Unpackaged-System.log -s -r junit -o $(artifactsDir)\TEST-AppInstallerCLI-Unpackaged-System.xml
-      workingDirectory: '$(buildOutDir)\AppInstallerCLITests'
+    workingDirectory: '$(buildOutDir)\AppInstallerCLITests'
+    condition: succeededOrFailed()
+
+  - powershell: |
+      Uninstall-WinGetPackage -Id Microsoft.Sysinternals.PsTools -Source winget
+    displayName: Clean up Sysinternals PsTools
     condition: succeededOrFailed()
 
   - task: PowerShell@2