Use vars to populate preview url

[nginx-jack]

Proposed changes

Use vars to populate preview url.
This will allow for forked versions of the repo to generate different preview urls without modifying the action file itself. (Mainly for internal use)

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[github-actions]

✅ Deploy Preview will be available once build job completes!

Name	Link
😎 Deploy Preview	https://frontdoor-test-docs.nginx.com/previews/docs/173/

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