Use bash to launch reclaim space script

See gh-42776

Author: philwebb

buildSrc/src/main/java/org/springframework/boot/build/test/DockerTestPlugin.java

@@ -124,7 +124,7 @@ private Provider<Exec> createReclaimDockerSpaceTask(Project project,
 			task.setDescription("Reclaims Docker space on CI.");
 			task.shouldRunAfter(DOCKER_TEST_TASK_NAME);
 			task.onlyIf(this::shouldReclaimDockerSpace);
-			task.executable("sh");
+			task.executable("bash");
 			task.args("-c",
 					project.getRootDir()
 						.toPath()