ci: add "PR action: merge" label to renovate bot PRs
ci: add "PR action: merge" label to renovate bot PRs

This label is expected to be added by authors after the review once they have made final edits to the PR (per contributing guidelines: https://github.com/angular/angular-cli/blob/master/CONTRIBUTING.md#-submitting-a-pull-request-pr). The bot will never make additional edits from a review, so it is always ready to merge. This adds the "PR action: merge" label so it will show up correctly in the caretaker search for PRs ready to merge (https://github.com/angular/angular-cli/blob/master/CONTRIBUTING.md#-submitting-a-pull-request-pr). Renovate PRs will still require review and passing status checks before becoming visible to caretakers.

angular/angular-cli,filipesilva/angular-cli,clydin/angular-cli,angular/angular-cli,clydin/angular-cli,ocombe/angular-cli,clydin/angular-cli,clydin/angular-cli,beeman/angular-cli,angular/angular-cli,catull/angular-cli,beeman/angular-cli,beeman/angular-cli,manekinekko/angular-cli,ocombe/angular-cli,filipesilva/angular-cli,Brocco/angular-cli,Brocco/angular-cli,catull/angular-cli,ocombe/angular-cli,catull/angular-cli,filipesilva/angular-cli,ocombe/angular-cli,filipesilva/angular-cli,manekinekko/angular-cli,Brocco/angular-cli,geofffilippi/angular-cli,geofffilippi/angular-cli,beeman/angular-cli,geofffilippi/angular-cli,geofffilippi/angular-cli,Brocco/angular-cli,Brocco/angular-cli,catull/angular-cli,angular/angular-cli,manekinekko/angular-cli,ocombe/angular-cli