Ensure authentication requirements are evaluated correctly.

Methods defined outside Groovy/Gradle DSL cannot be invoked (called) inside Groovy/Gradle DSL.

buildSrc/src/main/groovy/io/spring/gradle/convention/ArtifactoryPlugin.groovy

@@ -35,12 +35,14 @@ class ArtifactoryPlugin implements Plugin<Project> {
 		project.plugins.apply('com.jfrog.artifactory')
 
 		def artifactoryRepoKey = resolveRepositoryKey(project)
+		def authRequired = isAuthRequired(project)
+
 		project.artifactory {
 			contextUrl = 'https://repo.spring.io'
 			publish {
 				repository {
 					repoKey = artifactoryRepoKey
-					if (isAuthRequired(project)) {
+					if (authRequired) {
 						username = artifactoryUsername
 						password = artifactoryPassword
 					}