github actions: extract repo/branch names from PR message

github actions: extract repo/branch names from PR message

Create a "prep" build that parses libhtp, suricata-update and
suricata-verify repo and branch information from the pull
request message and turn these into artifacts that are
used by the builders

Examples:

libhtp-repo: https://github.com/OISF/libhtp
libhtp-branch: 0.5.x

suricata-verify-repo: http://github.com/OISF/suricata-verify
suricata-verify-branch: master

suricata-update-repo: http://github.com/OISF/suricata-update
suricata-update-branch: master

Other changes:
- checkout@v2 (faster)
- working directory cleanup