Since #39853, it is possible to deploy to multiple Firebase sites from a
single branch. In order to deploy to a site, we need to associate an
alias (`aio`) with a site. This is done via the `firebase target:apply`
command. However, when the command is called multiple times, it
associates the alias with many sites, which subsequently fails during
deployment ([example failure][1]), since the `firebase deploy` command
does not know what site to deploy to.
This commit fixes the deployment script by ensuring that any previous
association with the `aio` alias is cleared (via the
`firebase target:clear` command) before associating it with a new site.
[1]: https://circleci.com/gh/angular/angular/871020
PR Close#39948
f01c713ee2