Fix crash on remote-only action template outputs

[fmeum]

Fixes the following type of crash when the outputs of an action template are built with BwoB without being downloaded:

java.lang.RuntimeException: Unrecoverable error while evaluating node 'TargetCompletionKey{topLevelArtifactContext=com.google.devtools.build.lib.analysis.TopLevelArtifactContext@90504ed, actionLookupKey=ConfiguredTargetKey{label=//a:main, config=BuildConfigurationKey[ace1802e9ffc3691a893a40a89ff0d55ab85b5bb1a265fd1bb830b1af3f3a0e0]}, willTest=false}' (requested by nodes 'BuildDriverKey of ActionLookupKey: ConfiguredTargetKey{label=//a:main, config=BuildConfigurationKey[4e639ef1ee6b55f3683fb1281a60a624b3a6eb2ef19705d98d7e875872945deb]}')
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:551)
	at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:435)
	at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.compute(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.compute(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinTask$InterruptibleTask.exec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinTask.doExec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinPool.runWorker(Unknown Source)
	at java.base/java.util.concurrent.ForkJoinWorkerThread.run(Unknown Source)
Caused by: java.lang.IllegalStateException: generating action for artifact File:[[<execution_root>]bazel-out/darwin_arm64-fastbuild/bin]a/_objs/main/dir is not an ActionExecutionMetadata
	at com.google.common.base.Preconditions.checkState(Preconditions.java:603)
	at com.google.devtools.build.lib.remote.RemoteImportantOutputHandler.getGeneratingAction(RemoteImportantOutputHandler.java:191)
	at com.google.devtools.build.lib.remote.RemoteImportantOutputHandler.downloadArtifact(RemoteImportantOutputHandler.java:164)
	at com.google.devtools.build.lib.remote.RemoteImportantOutputHandler.ensureToplevelArtifacts(RemoteImportantOutputHandler.java:117)
	at com.google.devtools.build.lib.remote.RemoteImportantOutputHandler.processOutputsAndGetLostArtifacts(RemoteImportantOutputHandler.java:72)
	at com.google.devtools.build.lib.skyframe.CompletionFunction.informImportantOutputHandler(CompletionFunction.java:431)
	at com.google.devtools.build.lib.skyframe.CompletionFunction.compute(CompletionFunction.java:327)
	at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:471)

[fmeum]

@bazel-io fork 8.3.0

[fmeum]

@iancha1992 ready to merge

[iancha1992]

@bazel-io fork 8.5.0