The `pull_github_pr.sh` script has been fetching the username
from the owner of the source branch.
The owner of the branch is not always the author of the PR.
For example the branch might come from a fork managed by organization
or group of people.
This lead to having the author in merge commits refered to as `null`
(if the name was not set for the group) or it mentioned a name
not belonging to the author of the patch.
Instead looking for the owner of the source branch, the script should
look for the name of the PR's author.
Closesscylladb/scylladb#25363
b284961a95