Clone the bind9-qa repo to the project root in CI jobs

Clone the bind9-qa repo to the project root in CI jobs

Cloning to a stable location allows clearer handling of paths when
calling scripts from CI jobs.

`unit:gcc:tarball` and `system:gcc:tarball` do `cd bind-*` in
`before_script` which lead to the `bind9-qa` directory ending up in
a different place in exactly these two jobs and that made reasoning
about paths in `.system_test_common` and `.unit_test_common` tricky.

(cherry picked from commit 482c1cc72f0ed4b326a1cbdf85082f36f1a95c82)