Windows Terminal fails to instantiate Azure Shell when it is configured with an isolated VNET

# Environment

```none
Windows build number: Microsoft Windows [Version 10.0.18363.1379]
Windows Terminal version (if applicable): Version: 1.0.1401.0

Any other software? No
```

# Steps to reproduce

Configure Cloud Shell with isolated VNET settings per https://docs.microsoft.com/en-us/azure/cloud-shell/private-vnet
Validate correct Cloud Shell behavior with https://shell.azure.com
In Windows Terminal attempt to create a new session for Azure Cloud Shell

# Expected behavior

Cloud shell opens.

# Actual behavior

Cloud shell fails to instantiate with the messages:
You have not set up your cloud shell account yet. Please go to https://shell.azure.com to set it up.
[process exited with code 1]

Start by reproducing the failure using the isolated VNET configuration described in the Azure Cloud Shell documentation, then compare it with the successful shell.azure.com behavior. Trace the Windows Terminal Azure Cloud Shell session-creation path and determine why it reports that the account is not set up. Done means an isolated-VNET Cloud Shell session opens successfully in Windows Terminal without the setup error.

Written by the indexing model from the issue text.