code: public GitlabBuildVariable getBuildVariable(Integer projectId, String key)
              throws IOException {
          String tailUrl = GitlabProject.URL + "/" +
                  projectId +
                  GitlabBuildVariable.URL + "/" +
                  key;
          return retrieve().to(tailUrl, GitlabBuildVariable.class);
      }
docs: Gets build variable associated with a project and key.
  
  @param projectId The ID of the project.
  @param key       The key of the variable.
  @return A variable.
  @throws IOException on gitlab api call error