chore(ci): avoid unnecessarily running `grunt prepareDeploy` in `deploy-docs` CI job

Previously, the `grunt prepareDeploy` command was run in both the
`prepare-deployment` and `deploy-docs` CI jobs. The reason was that not
all files affected by `grunt prepareDeploy` were persisted to the
workspace across jobs.

More specifically, the command would affect files in the `deploy/` and
`scripts/docs.angularjs.org-firebase/` directories and also create a
`firebase.json` file at the root directory, but only the `deploy/`
directory was [persisted to the workspace][1].

This commit avoids unnecessarily running the `grunt prepareDeploy`
command in the `deploy-docs` CI job by ensuring that all affected files
will be persisted to the workspace in the `prepare-deployment` CI job,
which always runs before `deploy-docs`.

[1]: https://github.com/angular/angular.js/blob/295213df953766625462/.circleci/config.yml#L265

Closes #17060