chore: add toggle for enabling/disabling PR pipelines in Jenkinsfile [WPB-21880]#4571

MohamadJaara wants to merge 2 commits intodevelopwireapp/wire-android:developfrom

mo/chore/stop-the-jenkins-PR-runwireapp/wire-android:mo/chore/stop-the-jenkins-PR-runCopy head branch name to clipboard