Fix logic issue with MSI auth

[FlorianBracq]

Fix an issue with the MSI authentication.

For a workflow with:

• authentication to the end resource with application secret stored in a keyvault
• authentication to the keyvault with a managed identity

For the first pass (to authenticate against the KV) the value "client_id" is not set and properly retrieved from the environment.
However, once the creds are retrieved from the KV, an msi connection is also attempted with the client_id from the keyvault.

This fix aims to prevent that issue, and is more consistent with what was previously implemented:

    msi_kwargs = kwargs.copy()
     if AzureCredEnvNames.AZURE_CLIENT_ID in os.environ:
         msi_kwargs["client_id"] = os.environ[AzureCredEnvNames.AZURE_CLIENT_ID]