[release/9.0] Use dotnet-public feed for downloading NuGet.VerifyMicrosoftPackage (#16188)

Co-authored-by: Joey Robichaud <[email protected]>

Author: github-actions[bot]

eng/common/post-build/nuget-verification.ps1

@@ -30,7 +30,7 @@
 [CmdletBinding(PositionalBinding = $false)]
 param(
    [string]$NuGetExePath,
-   [string]$PackageSource = "https://api.nuget.org/v3/index.json",
+   [string]$PackageSource = "https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json",
    [string]$DownloadPath,
    [Parameter(ValueFromRemainingArguments = $true)]
    [string[]]$args