Get all pipeline Jobs in Jenkins Groovy Script

All pipeline jobs are instantces of org.jenkinsci.plugins.workflow.job.WorkflowJob. So you can get names of all Pipeline jobs using the following function

@NonCPS
def getPipelineJobNames() {
    Hudson.instance.getAllItems(org.jenkinsci.plugins.workflow.job.WorkflowJob)*.fullName 
}

Then you can use it this way

//...
def jobs = getPipelineJobNames()
if (job_to_build in jobs) {
    //....
}

try this syntax to get standard and pipeline jobs:

def jobs = Hudson.instance.getAllItems(hudson.model.Job.class)