Jenkinsfile: Fix presentation of the 'Check configuration' stage.

* Jenkinsfile ('Check configuration'): Rewrite stage as a script.
This makes the stage show as passed instead of skip in Blue Ocean,
which is more accurate.

Change-Id: I5be0cdc0c918521c3580b60261b98b9dff804876

Jenkinsfile

@@ -73,12 +73,15 @@ pipeline {
 
     stages {
         stage('Check configuration') {
-            when { not { expression { fileExists TARBALLS } } }
             steps {
+                script {
+                    if (!fileExists(TARBALLS)) {
                         error "The ${TARBALLS} directory does not exist. \
 See https://wiki.savoirfairelinux.com/wiki/Jenkins.jami.net#Configuration"
                     }
                 }
+            }
+        }
 
         stage('Fetch submodules') {
             steps {