Site-www: CI: site is deployed twice by two separate jobs in same build

[chalin]

Changes introduced by #1719 cause the site to be deployed twice, now that all jobs are passing. For example, consider build #3839; the site gets deployed for these two jobs:

• 3839.1:

Deploying application ...
Deploying to Firebase project: default

• 3839.6:
  Deploying application ...
  Deploying to Firebase project: default

cc @johnpryan @legalcodes @kwalrath

[chalin]

I can confirm, with #2017 merged and built via build 3844, that only the stable build job triggers a deploy action.

[chalin]

Build 3845 is the first cron job build. I confirm that the stable build job does not trigger a site deploy:

Skipping a deployment with the script provider because a custom condition was not met

Related: #1247