fix: update reference to language property in template

Update the reference to the `language` property in the 
`start-track-button.hbs` template to use the correct 
context variable. This change ensures that the button 
displays correctly based on the user's authentication 
status and the presence of a primer concept group.

Author: rohitpaulk

app/components/track-page/start-track-button.hbs

@@ -1,6 +1,6 @@
 <PrimaryButton class="mr-2 flex items-center" data-test-primary-start-track-button {{on "click" this.handleClicked}} @size="large" ...attributes>
   {{! If there's no primer concept group, the user will need to login to start the track }}
-  {{#if (and this.authenticator.isAnonymous (not this.args.language.primerConceptGroup))}}
+  {{#if (and this.authenticator.isAnonymous (not @language.primerConceptGroup))}}
     {{svg-jar "github" class="fill-current w-5 transform transition-all mr-2"}}
   {{/if}}