Turn off emails to kuksa-dev from PRs

[ghost]

Errors in build jobs of pull requests can also be seen from the Github page of the pull request. To reduce e-mails that are not informative to many people, it was therefore decided during the Kuksa technical exchange that build errors for pull requests should not trigger an e-mail to kuksa-dev, while build errors on master should still trigger an e-mail to kuksa-dev. This decision might be reverted / modified in the future.