Adds deploy attribution.

[mbleigh]

By adding the FIREBASE_DEPLOY_AGENT env variable we will be able to properly track attribution for GH Action deploys. See here.

[github-actions]

Deploy preview for 7750f08:

https://action-hosting-deploy-demo--pr18-mb-attribution-yq9u0iwl.web.app

_{(expires Wed, 16 Sep 2020 23:12:29 GMT)}

_{firebase-hosting-preview-action}

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[jhuleatt]

CLA bot got confused by my personal email

[jhuleatt]

Thanks @mbleigh! Excited to get some stats!