Fix NU5050 error in nuget pack

[xupiter]

An updated version of NuGet CLI (v5.10) was released in June, 2021 to address issues while packaging nuspec with duplicate files. For more context, see NuGet/Home#6941

This update introduced a build break for HtmlGenerator project, and users now keep getting NU5050 error in "nuget pack". The issue breaks the whole solution since it doesn't allow building SourceBrowser. Here's an example of what the error looks like:

       "C:\SourceBrowser\src\HtmlGenerator\HtmlGenerator.csproj" (default target) (2:7) ->

       (BuildNuGet target) -> 

         EXEC : error NU5050: Attempted to pack multiple files into the same location(s). The following destinations were used multiple times: tools\web\Microsoft.Build.dll, tools\web\Microsoft.Extensions.Configuration.UserSecrets.dll, tools\web\Microsoft.Extensions.Logging.Abstractions.dll, tools\web\Microsoft.Extensions.Primitives.dll, tools\web\System.Reactive.xml, tools\web\Microsoft.Build.Framework.dll, tools\web\Microsoft.Extensions.DependencyInjection.Abstractions.dll, tools\web\Microsoft.Extensions.Logging.Configuration.dll, tools\web\Microsoft.SourceBrowser.Common.dll, tools\web\System.ServiceProcess.ServiceController.dll, tools\web\Microsoft.Extensions.Configuration.Abstractions.dll, tools\web\Microsoft.Extensions.DependencyInjection.dll, tools\web\Microsoft.Extensions.Logging.Console.dll, tools\web\Microsoft.SourceBrowser.Common.pdb, tools\web\web.config, tools\web\Microsoft.Extensions.Configuration.Binder.dll, tools\web\Microsoft.Extensions.FileProviders.Abstractions.dll, tools\web\Microsoft.Extensions.Logging.Debug.dll, tools\web\Microsoft.SourceBrowser.SourceIndexServer.deps.json, tools\web\Microsoft.Extensions.Configuration.CommandLine.dll, tools\web\Microsoft.Extensions.FileProviders.Physical.dll, tools\web\Microsoft.Extensions.Logging.dll, tools\web\Microsoft.SourceBrowser.SourceIndexServer.dll, tools\web\Microsoft.Extensions.Configuration.dll, tools\web\Microsoft.Extensions.FileSystemGlobbing.dll, tools\web\Microsoft.Extensions.Logging.EventLog.dll, tools\web\Microsoft.SourceBrowser.SourceIndexServer.exe, tools\web\Microsoft.Extensions.Configuration.EnvironmentVariables.dll, tools\web\Microsoft.Extensions.Hosting.Abstractions.dll, tools\web\Microsoft.Extensions.Logging.EventSource.dll, tools\web\Microsoft.SourceBrowser.SourceIndexServer.pdb, tools\web\Microsoft.Extensions.Configuration.FileExtensions.dll, tools\web\Microsoft.Extensions.Hosting.dll, tools\web\Microsoft.Extensions.Options.ConfigurationExtensions.dll, tools\web\Microsoft.SourceBrowser.SourceIndexServer.runtimeconfig.json, tools\web\Microsoft.Extensions.Configuration.Json.dll, tools\web\Microsoft.Extensions.Hosting.WindowsServices.dll, tools\web\Microsoft.Extensions.Options.dll, tools\web\System.Reactive.dll [C:\SourceBrowser\src\HtmlGenerator\HtmlGenerator.csproj]

One way to fix that would be change download link for "nuget.exe" to get NuGet 5.9 (instead of the latest version).

[KirillOsenkov]

Thanks, fixed.