Enable token encryption when running on Linux

**Related command**

az login

**Is your feature request related to a problem? Please describe.**

Azure CLI has not yet enable token encryption when running on Linux even when MSAL now has implemented support for this.

**Describe the solution you'd like**

Azure CLI should store the token cache encrypted instead if plaintext on Linux

**Describe alternatives you've considered**

The current solution is to depend on file permissions 600 set the by CLI on the token cache file. However, encrypting it as well will improve the security posture.

**Additional context**

This is essentially the follow up on thread [https://github.com/AzureAD/microsoft-authentication-extensions-for-python/issues/106](https://github.com/AzureAD/microsoft-authentication-extensions-for-python/issues/106)