getSubscriptions() throws if user has signed in to two accounts

[StephenWeatherford]

1. run the script from Use vscode authentication instead of Azure Account extension Azure/bicep#15403 (comment) to install a version of the Bicep extension that uses the new @microsoft/vscode-azext-azureauth functionality
2. Using azureResourceGroups.login, log in to two different accounts in Azure. (Note: I did this because I was testing a different Azure account, but it wasn't obvious how to sign out of the first one, I assumed this would do it.)
3. Create a new main.bicep file and save it with:

resource storageaccount 'Microsoft.Storage/storageAccounts@2021-02-01' = {
  name: 'name'
  location: 'westus3'
  kind: 'StorageV2'
  sku: {
    name: 'Premium_LRS'
  }
}

4. in the editor's context menu for main.bicep, click "Deploy Bicep File..."
5. Click enter for default deployment name
   !) Next you should see a picklist of subscriptions, but you will instead see ,in the Output window, under "Bicep Operations", this error:

1:57:16 PM: Deployment failed for c:\Users\stephwe\Downloads\main.bicep. You are not signed in to an Azure account. Please sign in.
EXPECTED: should work

More details:

We call isSignedIn(), which finds that there's a session open for the first account and returns true. So then we call getSubscriptions(), which calls getTenants() on each account. The second getTenants() call throws because the second account doesn't have an open session:

getSubscriptionClient (vscode-bicep\node_modules\@microsoft\vscode-azext-azureauth\out\src\VSCodeAzureSubscriptionProvider.js:297)
getTenants (vscode-bicep\node_modules\@microsoft\vscode-azext-azureauth\out\src\VSCodeAzureSubscriptionProvider.js:80)
getSubscriptions (vscode-bicep\node_modules\@microsoft\vscode-azext-azureauth\out\src\VSCodeAzureSubscriptionProvider.js:138)