Fix nested submodule credential inheritance in CI

GitLab Runner 18.6+ externalizes Git configuration and doesn't
automatically inherit credentials to nested submodules. This caused
"Could not access submodule 'tests_api'" errors when git tried to
validate the nested tests_api submodule in btracker.

Fix:
- sync, setup-scripts-test: Use GIT_SUBMODULE_STRATEGY: none with
  manual non-recursive init (these jobs don't need tests_api)
- pattern-test: Use explicit credential inheritance via
  git -c "include.path=$(git -C $CI_PROJECT_DIR config include.path)"
  to properly init the nested tests_api submodule

See: https://docs.gitlab.com/ci/runners/git_submodules/