Fix the url of Cirrus failure notifications [ci skip]
The previous one was the url of this notification action, but we want to see a link to Cirrus CI.
This code follows https://cirrus-ci.org/guide/notifications/.
Also, head_branch was null only for pull requests from a fork, so we can use it for branch-based filtering, which is good.
Fix the url of Cirrus failure notifications [ci skip]
The previous one was the url of this notification action, but we want to
see a link to Cirrus CI.
This code follows https://cirrus-ci.org/guide/notifications/.
Also, head_branch was null only for pull requests from a fork, so we can
use it for branch-based filtering, which is good.