github-ci: use bundle.sh script for libhtp, suricata-update

github-ci: use bundle.sh script for libhtp, suricata-update

Update the GitHub CI workflow to use the bundle.sh script to pull in
Suricata-Update and libhtp. This means one less place where defaults
are hardcoded and can get out of sync.

This also simplifies the variable names that can be embedded in a pull
request message to use the same variable names that bundle.sh
expects. Of note, this removes the _PR variant, instead a branch name
of "pr/N" can be used to specify a PR.