chore: using constant instead of hardcode

initializer-backend/src/main/java/com/schbrain/archetype/initializer/maven/MavenUtils.java

@@ -9,6 +9,8 @@ import org.apache.maven.cli.MavenCli;
 
 import java.io.File;
 
+import static org.apache.maven.cli.MavenCli.MULTIMODULE_PROJECT_DIRECTORY;
+
 /**
  * @author liaozan
  * @since 2022/3/20
@@ -26,7 +28,7 @@ public class MavenUtils {
 
         String[] args = getArgs(param, outputDirectory);
 
-        System.getProperties().setProperty("maven.multiModuleProjectDirectory", "$M2_HOME");
+        System.getProperties().setProperty(MULTIMODULE_PROJECT_DIRECTORY, "$M2_HOME");
         mavenCli.doMain(args, null, System.out, System.err);
 
         log.info("Generate archetype project at {}", outputDirectory);