code: public File getBuildDir() {
          // we use the null check variant so that people can write true unit tests with a mock ItemParent
          // and without a JenkinsRule. Such tests are of limited utility as there is a high risk of hitting
          // some code that needs the singleton, but for persistence migration test cases it makes sense to permit
          Jenkins j = Jenkins.getInstanceOrNull();
          if (j == null) {
              return new File(getRootDir(), "builds");
          }
          return j.getBuildDirFor(this);
      }
docs: Directory for storing {@link Run} records.
  <p>
  Some {@link Job}s may not have backing data store for {@link Run}s, but
  those {@link Job}s that use file system for storing data should use this
  directory for consistency.
  
  @see RunMap