fix: Moved Gradle TestKit directory to separate OS temp directory

In 748292c the GradleRuner used the
temporary directory of the current test (projectDir) which comes from
JUnit's `@TempDir`. When JUnit finishes its test it tries to cleanupt
the temporary folder, however the daemon may still be running and lock
files under this folder.

With the change each fixture still gets its own isolated testkit dir
to ensure the daemon isolation preserved.

gradle/gradle#12535

Author: bric3

buildSrc/src/test/kotlin/datadog/gradle/plugin/GradleFixture.kt

@@ -6,13 +6,23 @@ import org.gradle.testkit.runner.UnexpectedBuildResultException
 import org.intellij.lang.annotations.Language
 import org.w3c.dom.Document
 import java.io.File
+import java.nio.file.Files
 import javax.xml.parsers.DocumentBuilderFactory
 
 /**
  * Base fixture for Gradle plugin integration tests.
  * Provides common functionality for setting up test projects and running Gradle builds.
  */
 internal open class GradleFixture(protected val projectDir: File) {
+  // Keep test-kit caches outside @TempDir so Gradle daemon file-locks on
+  // .testkit/caches/*/transforms don't cause JUnit @TempDir cleanup failures
+  // (DirectoryNotEmptyException).  Each fixture still gets its own testkit dir,
+  // ensuring daemon isolation (fresh MAVEN_REPOSITORY_PROXY per test).
+  // Cleanup is left to the OS (/tmp is ephemeral on CI containers).
+  private val testKitDir: File by lazy {
+    Files.createTempDirectory("gradle-testkit-").toFile()
+  }
+
   /**
    * Runs Gradle with the specified arguments.
    *
@@ -23,12 +33,7 @@ internal open class GradleFixture(protected val projectDir: File) {
    */
   fun run(vararg args: String, expectFailure: Boolean = false, env: Map<String, String> = emptyMap()): BuildResult {
     val runner = GradleRunner.create()
-      // Use a testkit dir scoped to this fixture's projectDir. The Tooling API always uses a
-      // daemon and ignores org.gradle.daemon=false. By giving each test its own testkit dir,
-      // we force a fresh daemon per test — ensuring withEnvironment() vars (e.g.
-      // MAVEN_REPOSITORY_PROXY) are correctly set on the daemon JVM and not inherited from
-      // a previously-started daemon with a different test's environment.
-      .withTestKitDir(file(".testkit"))
+      .withTestKitDir(testKitDir)
       .withPluginClasspath()
       .withProjectDir(projectDir)
       .withEnvironment(System.getenv() + env)