Set withTestKitDir per Gradle version

Attempt to fix cache locking issues by setting a unique `withTestKitDir`
directory for each tested Gradle version.

Closes gh-24993

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/java/org/springframework/boot/gradle/testkit/GradleBuild.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012-2020 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -163,6 +163,7 @@ public class GradleBuild {
 		if (this.gradleVersion != null) {
 			gradleRunner.withGradleVersion(this.gradleVersion);
 		}
+		gradleRunner.withTestKitDir(getTestKitDir());
 		List<String> allArguments = new ArrayList<>();
 		allArguments.add("-PbootVersion=" + getBootVersion());
 		allArguments.add("--stacktrace");
@@ -172,6 +173,13 @@ public class GradleBuild {
 		return gradleRunner.withArguments(allArguments);
 	}
 
+	private File getTestKitDir() {
+		File temp = new File(System.getProperty("java.io.tmpdir"));
+		String username = System.getProperty("user.name");
+		String gradleVersion = (this.gradleVersion != null) ? this.gradleVersion : "default";
+		return new File(temp, ".gradle-test-kit-" + username + "-" + getBootVersion() + "-" + gradleVersion);
+	}
+
 	public File getProjectDir() {
 		return this.projectDir;
 	}