Fix cross-runner path issues in dotenv artifacts

Use relative paths in dotenv files for variables that store file paths.
GitLab CI expands variables at load time, so absolute paths break when
jobs run on different runner slots.

Changes:
- npm_pack_package.sh: Store PACKAGE_SOURCE_DIR and BUILT_PACKAGE_PATH
  as ${CI_PROJECT_DIR}/relative/path for runtime expansion
- npm_projects.gitlab-ci.yml: Fix BUILT_JSON_SWAGGER_FILE in
  .filter_out_swagger_json template