### CI Link

https://gradle-enterprise.elastic.co/s/yp56iltyyinha

### Repro line

./gradlew ':x-pack:plugin:ilm:qa:multi-node:javaRestTest' --tests "org.elasticsearch.xpack.ilm.ChangePolicyforIndexIT.testChangePolicyForIndex" -Dtests.seed=8825024EAE3089DD -Dtests.locale=en-PH -Dtests.timezone=Asia/Karachi -Druntime.java=16

### Does it reproduce?

No

### Applicable branches

7.17 (probably all 8+)

### Failure history

_No response_

### Failure excerpt

This is rather concerning
```
java.lang.AssertionError: policy step registry cache failed sanity check
	at org.elasticsearch.xpack.ilm.PolicyStepsRegistry.getStep(PolicyStepsRegistry.java:378) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleRunner.getCurrentStep(IndexLifecycleRunner.java:108) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleRunner.runPolicyAfterStateChange(IndexLifecycleRunner.java:430) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleService.triggerPolicies(IndexLifecycleService.java:416) ~[?:?]
	at org.elasticsearch.xpack.ilm.IndexLifecycleService.clusterChanged(IndexLifecycleService.java:319) ~[?:?]
	at org.elasticsearch.cluster.service.ClusterApplierService.callClusterStateListener(ClusterApplierService.java:573) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.callClusterStateListeners(ClusterApplierService.java:559) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.applyChanges(ClusterApplierService.java:519) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.runTask(ClusterApplierService.java:428) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService.access$000(ClusterApplierService.java:56) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.cluster.service.ClusterApplierService$UpdateTask.run(ClusterApplierService.java:154) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingRunnable.run(ThreadContext.java:718) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.runAndClean(PrioritizedEsThreadPoolExecutor.java:262) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at org.elasticsearch.common.util.concurrent.PrioritizedEsThreadPoolExecutor$TieBreakingPrioritizedRunnable.run(PrioritizedEsThreadPoolExecutor.java:225) ~[elasticsearch-7.17.2-SNAPSHOT.jar:7.17.2-SNAPSHOT]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1130) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:630) ~[?:?]
	at java.lang.Thread.run(Thread.java:831) [?:?]
[2022-03-15T09:34:09.657115416Z] [BUILD] Stopping node
```
x-pack/plugin/ilm/src/test/java/org/elasticsearch/xpack/ilm/PolicyStepsRegistryTests.java
private static final Step.StepKey MOCK_STEP_KEY = new Step.StepKey("mock", "mock", "mock");
    private static final NamedXContentRegistry REGISTRY = new NamedXContentRegistry(new IndexLifecycle(Settings.EMPTY).getNamedXContent());
    private IndexMetadata emptyMetadata(Index index) {
        return IndexMetadata.builder(index.getName())
            .settings(settings(Version.CURRENT))
            .numberOfShards(randomIntBetween(1, 5))
            .numberOfReplicas(randomIntBetween(0, 5))
            .build();
    }
    public void testGetFirstStep() {
        String policyName = randomAlphaOfLengthBetween(2, 10);
        Step expectedFirstStep = new MockStep(MOCK_STEP_KEY, null);
        Map<String, Step> firstStepMap = Collections.singletonMap(policyName, expectedFirstStep);
        PolicyStepsRegistry registry = new PolicyStepsRegistry(null, firstStepMap, null, NamedXContentRegistry.EMPTY, null, null);
        Step actualFirstStep = registry.getFirstStep(policyName);
        assertThat(actualFirstStep, sameInstance(expectedFirstStep));
    }
    public void testGetFirstStepUnknownPolicy() {
        String policyName = randomAlphaOfLengthBetween(2, 10);
        Step expectedFirstStep = new MockStep(MOCK_STEP_KEY, null);
        Map<String, Step> firstStepMap = Collections.singletonMap(policyName, expectedFirstStep);
        PolicyStepsRegistry registry = new PolicyStepsRegistry(null, firstStepMap, null, NamedXContentRegistry.EMPTY, null, null);
        Step actualFirstStep = registry.getFirstStep(policyName + "unknown");
        assertNull(actualFirstStep);
    }
    public void testGetStep() {