chore(CircleCI): fix `deploy-docs` job

Since #17039, our docs Firebase functions' `package.json` specifies a
`node` engine version. This is required for configuring which version of
Node.js should Firebase use to execute the functions. However, since
Firebase is using an older version of Node.js than the one we use to
build the AngularJS project, yarn would error due to incompatible
Node.js engine versions ([example failure][1]).

This commit avoids the error by running yarn with the `--ignore-engines`
option.

[1]: https://app.circleci.com/pipelines/github/angular/angular.js/214/
     workflows/ad2e9baf-7249-467d-bc71-bd98e6cd922c/jobs/2247