DAOS-16716 ci: Set reference build for PRs (#15337)

Release branch PRs should use the release branch build
instead of master branch build for NLT reference

Signed-off-by: Jeff Olivier <jeffolivier@google.com>

Jenkinsfile

@@ -801,6 +801,7 @@ pipeline {
                             unitTestPost artifacts: ['nlt_logs/'],
                                          testResults: 'nlt-junit.xml',
                                          always_script: 'ci/unit/test_nlt_post.sh',
+					 referenceJobName: 'daos-stack/daos/release%252F2.6',
                                          valgrind_stash: 'el8-gcc-nlt-memcheck'
                             recordIssues enabledForFailure: true,
                                          failOnError: false,