Update Checkstyle in Gradle build

Fixes the following failing CI error to recent Gradle upgrade.

* What went wrong:

Execution failed for task ':practice:zipper:checkstyleTest'.

> A failure occurred while executing org.gradle.api.plugins.quality.internal.CheckstyleAction

   > An unexpected error occurred configuring and executing Checkstyle.

      > Unable to create Root Module: config {/home/runner/work/java/java/exercises/practice/zipper/checkstyle.xml}.

Author: kahgoh

exercises/build.gradle

@@ -79,7 +79,7 @@ subprojects {
     // configuration of the linter
     checkstyle {
       toolVersion '10.7.0'
-      configFile file("checkstyle.xml")
+      configFile file("$rootDir/checkstyle.xml")
       sourceSets = [project.sourceSets.main, project.sourceSets.test]
     }