scripts/pull_github_pr.sh: don't guess git remote name
scripts/pull_github_pr.sh: don't guess git remote name

The script assumes the remote name is "origin", a fair
assumption, but not universally true. Read it from configuration
instead of guessing it.

Closes #9423

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla