DAOS-16716 ci: Set reference build for PRs (#15379)

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

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

Jenkinsfile

@@ -1038,7 +1038,7 @@ pipeline {
                     }
                     post {
                         always {
-                            discoverGitReferenceBuild referenceJob: 'daos-stack/daos/master',
+                            discoverGitReferenceBuild referenceJob: 'daos-stack/daos/release%252F2.6',
                                                       scm: 'daos-stack/daos',
                                                       requiredResult: hudson.model.Result.UNSTABLE
                             recordIssues enabledForFailure: true,