[Jenkins] Mark build as unstable if build jobs were skipped.

Helps to identify that not everything was built. Prevents empty
artifacts when browsing to
https://jenkins.opengeosys.org/job/ufz/job/ogs/job/master/lastSuccessfulBuild/artifact/

Jenkinsfile

@@ -77,6 +77,9 @@ pipeline {
               }
             }
           }
+          if(!stage_required.build || !stage_required.full) {
+            currentBuild.result='UNSTABLE'
+          }
         }
       }
       // Mark build as NOT_BUILT when [ci skip] commit message was found