This change forces ansible to decrypt the variable with ansible-vault if encrypted, to avoid the error '{"msg": "Unexpected templating type error occurred on ({{ matrix_synapse_macaroon_secret_key | password_hash('sha512') }}): secret must be unicode or bytes, not ansible.parsing.yaml.objects.AnsibleVaultEncryptedUnicode"}'
Every other variable in the playbook was found to have no problems with encryption.
The change has no negative impact on non-encrypted matrix_synapse_macaroon_secret_key.