gha/changelog/update/v2 action is broken when changelog sits at the project root for a mono repository

[NiklasRosenstein]

The action is supposed to find all changelog files, look at the newly added entries and inject the current PR's URL if they don't already have a PR URL set. However, this currently fails on kraken-build/kraken#141 because of this error:

Job: https://github.com/kraken-build/kraken/actions/runs/7202421349/jb/19620503985

We do by default treat the .changelog folder in the root of the project as read-only for a mono repository, but this is supposed to be configurable via the changelog.enabled = true setting in slap.toml, however this doesn't have any effect on the action.