Use temp directory when path of build directory is too long

Some gRPC-related tests fail on Windows when the path becomes too
long. The failure is somewhat cryptic:

> protoc: stdout: . stderr: --grpc_out: protoc-gen-grpc: The system cannot find the path specified.

The failure occurs when the path to the protoc-gen-grpc-java
executable gets too long. Our Gradle plugin's tests configure
TestKit to use a directory beneath spring-boot-gradle-plugin/build
and this can result in the path being too long.

This commit updates GradleBuild to use a directory beneath
java.io.tmpdir when the tests are running on Windows and the
absolute path of spring-boot-gradle-plugin/build 80 characters or
more in length.

Closes gh-51169

Author: wilkinsona

test-support/spring-boot-gradle-test-support/src/main/java/org/springframework/boot/testsupport/gradle/testkit/GradleBuild.java

@@ -210,12 +210,25 @@ private void copyTransformedScript(String script, File destination) throws IOExc
 	}
 
 	private File getTestKitDir() {
-		File build = this.buildOutput.getRootLocation();
-		File testKitRoot = new File(build, "gradle-test-kit");
+		File testKitRoot = getTestKitRoot();
 		String gradleVersion = (this.gradleVersion != null) ? this.gradleVersion : "default";
 		return new File(testKitRoot, gradleVersion).getAbsoluteFile();
 	}
 
+	private File getTestKitRoot() {
+		File build = this.buildOutput.getRootLocation();
+		if (isWindows() && build.getAbsolutePath().length() >= 80) {
+			return new File(System.getProperty("java.io.tmpdir"), "gradle-test-kit");
+		}
+		else {
+			return new File(build, "gradle-test-kit");
+		}
+	}
+
+	private boolean isWindows() {
+		return File.separatorChar == '\\';
+	}
+
 	public File getProjectDir() {
 		return this.projectDir;
 	}