Generate release notes job fails on tag push events

[klutchell]

Flowzone supports tag push events for repos not using versionist. However this flow is currently broken in the Generate release notes job step when it presumes that it is a PR event of some kind.

Example failed run: https://github.com/klutchell/unbound-docker/actions/runs/11168240103/job/31046215212#step:3:86

logs_29184104979.zip

+ set -x
++ gh pr view --json title --jq .title
[git symbolic-ref --quiet HEAD]
could not determine current branch: failed to run git: fatal: not a git repository (or any of the parent directories): .git

+ pr_title=
Error: Process completed with exit code 1.