ci: cleanup workspaces after every build to save space

Signed-off-by: Jakub Sokołowski <jakub@status.im>

ci/Jenkinsfile.prs

@@ -75,6 +75,7 @@ pipeline {
   post {
     success { script { github.notifyPR(true) } }
     failure { script { github.notifyPR(false) } }
+    always { cleanWs() }
   } // post
 } // pipeline