[9.18] chg: ci: Add commit link and diff to RPM build job logs

The output of update_rpms.py is terse, making it difficult to verify its
actions.  Add a commit link and "git show" output to the log of every CI
job running the update_rpms.py script in "build" mode to facilitate
double-checking its actions.

Backport of MR !11828

Merge branch 'backport-michal/add-commit-link-and-diff-to-rpm-build-job-logs-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!12000