[9.18] fix: ci: Do not evaluate $CI_PROJECT_DIR in generate-stress-test-configs.py

[9.18] fix: ci: Do not evaluate $CI_PROJECT_DIR in generate-stress-test-configs.py

GitLab CI Runner's $builds_dir variable is set to "/builds" by default.
For technical reasons, the FreeBSD Runners, using the "instance"
executor, sets the path differently.

The value of $CI_PROJECT_DIR is based on $builds_dir, so if the
generate-stress-test-configs.py script generates jobs with
$CI_PROJECT_DIR (or variables like $INSTALL_PATH that are based on it)
evaluated, it is calcified to whatever was the value in the particular
environment, disregarding the FreeBSD "instance" executor specifics in
the child pipeline.

Instead of evaluating $CI_PROJECT_DIR in the script, evaluate it in the
runtime environment.

Backport of MR !10075

Merge branch 'backport-mnowak/fix-CI_PROJECT_DIR-variable-evaluation-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!10078