Use Jenkins credentials binding for Nexus snapshot authentication

Jenkinsfile

@@ -3,7 +3,13 @@ pipeline {
     stages {
         stage('Gradle') {
             steps {
-                sh './gradlew build -PalttdSnapshotUsername=$NEXUS_CREDS_USR -PalttdSnapshotPassword=$NEXUS_CREDS_PSW'
+                withCredentials([usernameColonPassword(credentialsId: 'alttd-snapshot-user', usernameVariable: 'USERNAME', passwordVariable: 'PASSWORD')]) {
+                sh '''
+                  set +x
+                  chmod +x gradlew
+                  ./gradlew build -PalttdSnapshotUsername=$USERNAME -PalttdSnapshotPass=$PASSWORD
+                '''
+                }
             }
         }
         stage('Archive') {