Rename AZURE_POD_IDENTITY_TOKEN_URL -> AZURE_POD_IDENTITY_AUTHORITY_HOST

[chlowell]

The variable should be just the scheme and host of the URL, e.g. "http://169.254.169.254" (the default). ManagedIdentityCredential should add its default path ("/metadata/identity/oauth2/token")

See e.g. Azure/azure-sdk-for-python/pull/19867

[g2vinay]

Resolved and shipped.