Is there a way to add an "unsubscribe" link in the email variation?

Yes, you can. Simply click the unsubscribe attribute button in the email editor while editing your email variation and an unsubscribe footer will be added to your email.
See screenshot:

The "here" in the footer is automatically linked to {{=unsubscribe}} attribute

You can also link any text or phrase to the {{=unsubscribe}} attribute

1. Highlight the word or phrase (or image) you want to use as the unsubscribe link.

2. Click the “Insert/Edit Link” button

3. Enter {{=unsubscribe}} in the dialogue box and click “OK”

The {{=unsubscribe}} attribute automatically transforms to a unique URL for every emailed prospect so when they click the link, the prospect will no longer receive any follow-ups and will be tagged as “unsubscribed”.