fixed retrieval of nexus credentials

Jenkinsfile

@@ -17,7 +17,8 @@ pipeline {
             steps{
                 withGradle {
                     sh "chmod +x ./gradlew"
-                    sh "./gradlew ${gradleParams} -x test clean build"
+                    sh "./gradlew ${gradleParams} clean"
+                    sh "./gradlew ${gradleParams} -x test build"
                 }
             }
         }

build_publish.gradle

@@ -11,9 +11,11 @@ publishing {
             def releasesRepoUrl = 'http://oss.sonatype.org/service/local/staging/deploy/maven2/'
             url = project.hasProperty('release') && !version.endsWith('SNAPSHOT') ? releasesRepoUrl : snapshotsRepoUrl
 
-            credentials {
-                username System.getenv('mavenCentralUsername')
-                password System.getenv('mavenCentralPassword')
+            if (project.hasProperty('mavenCentralUsername') && project.hasProperty('mavenCentralPassword')) {
+                credentials {
+                    username project.property("mavenCentralUsername") as String
+                    password project.property("mavenCentralPassword") as String
+                }
             }
         }
     }