Attributes are not rendering correctly in tab titles

[kilfoyle]

Describe the bug

When I use attributes in tab titles, like this:

::::{tab-set}

:::{tab-item} {{ess}}

...

:::{tab-item} {{ece}}

The attributes are not interpolated:

Expected behavior

It would rock 🤘if we could see this:

Steps to reproduce

Create a tab set that contains attributes. In this example, {{ess}} and {{ece}}.

::::{tab-set}

:::{tab-item} {{ess}}
1. [Get a free trial](https://cloud.elastic.co/registration?page=docs&placement=docs-body).
2. Log into [Elastic Cloud](https://cloud.elastic.co?page=docs&placement=docs-body).
3. Select **Create deployment**.
4. Give your deployment a name. You can leave all other settings at their default values.
5. Select **Create deployment** and save your Elastic deployment credentials. You need these credentials later on.
6. When the deployment is ready, click **Continue** and a page of **Setup guides** is displayed. To continue to the deployment homepage click **I’d like to do something else**.

Prefer not to subscribe to yet another service? You can also get Elasticsearch Service through [AWS, Azure, and GCP marketplaces](../../../deploy-manage/deploy/elastic-cloud/subscribe-from-marketplace.md).
:::

:::{tab-item} {{ece}}
1. Log into the Elastic Cloud Enterprise admin console.
2. Select **Create deployment**.
3. Give your deployment a name. You can leave all other settings at their default values.
4. Select **Create deployment** and save your Elastic deployment credentials. You need these credentials later on.
5. When the deployment is ready, click **Continue** and a page of **Setup guides** is displayed. To continue to the deployment homepage click **I’d like to do something else**.
:::

::::

Tooling

• docs-builder
• migration tooling
• I'm not sure

[lcawl]

FYI This isn't working yet in #451

[shainaraskas]

@lcawl these weren't working over here because the variables are set per repository - the docs-builder repo was missing the subs.