Install yarn

Summary:
yarn is now used by the site builder packaged in the
ZURB WebApp Template as a npm alternative.

As such, when we update static sites
like infra.nasqueron.org or daeghrefn.nasqueron.org
we switch from npm to yarn.

That requires CD jobs to build with yarn.

Test Plan: yarn

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1900