Dynamic extension install not working as advertised in 2.16

> ### `az feedback` auto-generates most of the information requested below, as of CLI version 2.0.62

**Describe the bug**

az cli dynamic install should work and install uninstalled extensions on first command run; but it does not.

**To Reproduce**

- install az cli 2.16
- Run `az aks pod-identity add`

**Expected behavior**

shoulnd't be prompted for extension install

**Environment summary**

**Additional context**

```
bash-5.0# az aks pod-identity add
The command requires the extension aks-preview. Do you want to install it now? The command will continue to run after the extension is installed. (Y/n): ^Cbash-5.0#
bash-5.0#
bash-5.0#
bash-5.0# az --version
azure-cli 2.16.0 *
core 2.16.0 *
telemetry 1.0.6
Python location '/usr/bin/python3'
Extensions directory '/root/.azure/cliextensions'
Python (Linux) 3.8.10 (default, May 6 2021, 06:30:44)
[GCC 9.3.0]
Legal docs and information: aka.ms/AzureCliLegal

You have 2 updates available. Consider updating your CLI installation with 'az upgrade'
Please let us know how we are doing: https://aka.ms/azureclihats
and let us know if you're interested in trying out our newest features: https://aka.ms/CLIUXstudy

```