Update build.gradle and add checkstyle.xml

1. Update setting of 'scripts' as sourceSet.
   Original method caused issues with checkstyle.

2. Update checkstyle to use config/checkstyle/checkstyle.xml file, which
   is adapted from google-checks.xml.
   This allows for more granular control over checkstyle configurations.

Author: euchangxian

build.gradle

@@ -1,21 +1,13 @@
-/*
- * This file was generated by the Gradle 'init' task.
- *
- * This is a general purpose Gradle build.
- * To learn more about Gradle by exploring our Samples at https://docs.gradle.org/8.3/samples
- */
 plugins {
     id 'java'
     id "com.adarshr.test-logger" version "3.2.0"
     id "checkstyle"
 }
 
-sourceSets {
-    main {
-        java {
-            srcDirs = ['src/main/java', 'scripts']
-        }
-    }
+sourceSets.main.java.srcDirs += ['scripts']
+
+checkstyle {
+    toolVersion = '10.2'
 }
 
 repositories {
@@ -32,9 +24,3 @@ java {
         languageVersion = JavaLanguageVersion.of(11)
     }
 }
-
-checkstyle {
-    def archive = configurations.checkstyle.resolve().find { it.name.startsWith("checkstyle") }
-    config = resources.text.fromArchiveEntry(archive, "google_checks.xml")
-}
-