Test on GitHub Actions with the real Azure Blob Storage if needed

[dmikurube]

For the time being, embulk-input-azure_blob_storage is not tested with the real Azure Blob Storage. (The environment variables AZURE_ACCOUNT_NAME, AZURE_ACCOUNT_KEY, and AZURE_CONTAINER need to be set.)

The Embulk project may want to have an Azure account maintained by the community.

Or, we may want to test with a kind of compatible local storage, but note that they may not be compatible.