Use pull request number instead of branch name for branch deployments

[cortadocodes]

Bug report

What is the current behavior?

Many branch names are longer than the limit set by Google Cloud Run for service names when they're used as the revision tag, which makes deployment fail. We have to manually truncate branches in workflows that use this action to make up for it.

What is the expected behavior?

No truncation is needed and branch deployments just work.

[cortadocodes]

We may not need this because:

• Workflows triggered by pull requests are already in a short enough form that uses pull request number
• We can restrict push triggers to only run on main
• We might opt to replace automatic pull request triggers with manual workflow dispatches where the user can choose the revision tag

[thclark]

It wouldn't hurt to simply add a pr_number to the outputs.