Ubuntu 21.04 Does Not Provide /usr/bin/env python

Hi Team

When trying to install the Linux Diagnostic extension on Ubuntu 21.04, we are experiencing the following errors

```log
Error: Code="VMExtensionHandlerNonTransientError" Message="The handler for VM extension type 'Microsoft.Azure.Diagnostics.LinuxDiagnostic' has reported terminal failure for VM extension 'Bastion_StorageExtension' with error message: '[ExtensionOperationError] Non-zero exit code: 127, /var/lib/waagent/Microsoft.Azure.Diagnostics.LinuxDiagnostic-4.0.25/diagnostic.py -install
[stdout]
[stderr]
Running scope as unit: install_9b861366-ae3a-44ad-b9d9-695358c34c43.scope
/usr/bin/env: ‘python’: No such file or directory
'.\r\r
'Install handler failed for the extension. More information on troubleshooting is available at https://aka.ms/VMExtensionLinuxDiagnosticsTroubleshoot'"
```

Appreciate how to overcome this, and whether there will be releases in the future to support Ubuntu 21.04.

### Related Issue

- https://github.com/Azure/azure-linux-extensions/issues/1284
- https://github.com/Azure/azure-linux-extensions/issues/1296

Start with the diagnostic.py -install entry point and review related issues 1284 and 1296 for Ubuntu compatibility context. Reproduce the Linux Diagnostic extension installation failure on Ubuntu 21.04 and verify that installation completes without the missing-python error.

Written by the indexing model from the issue text.