Suppress running VS installer

Author: vmoroz

.ado/templates/prepare-build-env.yml

@@ -31,6 +31,19 @@ parameters:
     # invoked. Example: ['RNTesterApp-Fabric', 'Playground'].
 
 steps:
+  # The VS Installer's background auto-update service otherwise wakes up mid-build and
+  # downloads VS updates from the MS CDN, which trips the network isolation policy.
+  # Follow-up: bake this into the agent image so it doesn't have to run per job.
+  - pwsh: |
+      foreach ($key in @(
+          'HKLM:\SOFTWARE\Microsoft\VisualStudio\Setup',
+          'HKLM:\SOFTWARE\Policies\Microsoft\VisualStudio\Setup')) {
+        New-Item -Path $key -Force | Out-Null
+        New-ItemProperty -Path $key -Name BackgroundDownload -PropertyType DWord -Value 0 -Force | Out-Null
+      }
+      Get-Process -Name BackgroundDownload -ErrorAction SilentlyContinue | Stop-Process -Force -ErrorAction SilentlyContinue
+    displayName: Disable VS Installer background download
+
   # The commit tag in the nuspec requires that we use at least nuget 5.8 (because things break with nuget versions before and Vs 16.8 or later)
   - task: NuGetToolInstaller@1
     displayName: Set NuGet version