Publish to AWS Lambda fails with an error "Unable to load the service index"

**Describe the bug**
On AWS Serverless application running .NET Core 3.1 when one right clicks the project and chooses "Publish to AWS Lambda" there is an error accessing ADO Nuget Feed - Unable to load the service index for source - Response status code does not indicate success: 401 (Unauthorized).

**To Reproduce**
1. Right click Lambda project in Visual Studio 2019
2. Choose Publish to AWS Lambda
3. Fill the detail required - stack, bucket, parameters, etc
4. Click Publish button5.
**Expected behavior**
AWS stack should be created or updated

**Screenshots**

Screenshot and log are attached.
**Computer (please complete the following information):**

- Windows Version: Windows 10 Enterprise 64 bit
- Visual Studio Version: VS 2019 Version 16.10.4
- AWS Toolkit for Visual Studio Version: 1.21.2.0

**Additional context**
Started a few weeks ago. Solution compiles, tests run, I am able to publish to a folder without an issue.
[log.txt](https://github.com/aws/aws-toolkit-visual-studio/files/6888714/log.txt)

Start with the Visual Studio 2019 “Publish to AWS Lambda” flow for the .NET Core 3.1 project and review the attached log.txt, focusing on the 401 response from the ADO NuGet feed. Reproduce the publish steps with the reported AWS Toolkit and Visual Studio versions. Done means the stack can be created or updated without the service-index authorization error.

Written by the indexing model from the issue text.