Only get Kotlin plugin's version once we know we need it

Fixes gh-26946

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/plugin/KotlinPluginAction.java

@@ -33,9 +33,9 @@ class KotlinPluginAction implements PluginApplicationAction {
 
 	@Override
 	public void execute(Project project) {
-		String kotlinVersion = project.getPlugins().getPlugin(KotlinPluginWrapper.class).getKotlinPluginVersion();
 		ExtraPropertiesExtension extraProperties = project.getExtensions().getExtraProperties();
 		if (!extraProperties.has("kotlin.version")) {
+			String kotlinVersion = project.getPlugins().getPlugin(KotlinPluginWrapper.class).getKotlinPluginVersion();
 			extraProperties.set("kotlin.version", kotlinVersion);
 		}
 		enableJavaParametersOption(project);