Regression in sandbox stashing

### Description of the bug:

`bazel test //...` in with_cfg.bzl started crashing with last_green on Linux:
```
FATAL: bazel crashed due to an internal error. Printing stack trace:
java.lang.RuntimeException: Unrecoverable error while evaluating node 'UnshareableActionLookupData{actionLookupKey=ConfiguredTargetKey{label=//builder_reuse:first_cc_test, config=BuildConfigurationKey[46e1fa485d3659317264237218b8717064105f5afbe41b393d8e487d11b20ad3]}, actionIndex=5}' (requested by nodes 'TestCompletionKey{configuredTargetKey=ConfiguredTargetKey{label=//builder_reuse:first_cc_test, config=BuildConfigurationKey[46e1fa485d3659317264237218b8717064105f5afbe41b393d8e487d11b20ad3]}, topLevelArtifactContext=TopLevelArtifactContext[runTestsExclusively=false, outputGroups=[_hidden_top_level_INTERNAL_, _validation, default, temp_files_INTERNAL_], failOnUnknownOutputGroups=false, forRunCommand=false], exclusiveTesting=false}')
at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:552)
at com.google.devtools.build.lib.concurrent.AbstractQueueVisitor$WrappedRunnable.run(AbstractQueueVisitor.java:493)
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.NullPointerException
at com.google.common.base.Preconditions.checkNotNull(Preconditions.java:957)
at com.google.devtools.build.lib.vfs.Path.getRelative(Path.java:133)
at com.google.devtools.build.lib.sandbox.SandboxStash.takeStashedSandboxInternal(SandboxStash.java:185)
at com.google.devtools.build.lib.sandbox.SandboxStash.takeStashedSandbox(SandboxStash.java:129)
at com.google.devtools.build.lib.sandbox.SymlinkedSandboxedSpawn.filterInputsAndDirsToCreate(SymlinkedSandboxedSpawn.java:87)
at com.google.devtools.build.lib.sandbox.AbstractContainerizingSandboxedSpawn.createFileSystem(AbstractContainerizingSandboxedSpawn.java:133)
at com.google.devtools.build.lib.sandbox.AbstractSandboxSpawnRunner.runSpawn(AbstractSandboxSpawnRunner.java:174)
at com.google.devtools.build.lib.sandbox.AbstractSandboxSpawnRunner.exec(AbstractSandboxSpawnRunner.java:139)
at com.google.devtools.build.lib.exec.AbstractSpawnStrategy.exec(AbstractSpawnStrategy.java:137)
at com.google.devtools.build.lib.exec.AbstractSpawnStrategy.exec(AbstractSpawnStrategy.java:97)
at com.google.devtools.build.lib.exec.SpawnStrategyResolver.exec(SpawnStrategyResolver.java:46)
at com.google.devtools.build.lib.exec.StandaloneTestStrategy.runTestAttempt(StandaloneTestStrategy.java:664)
at com.google.devtools.build.lib.exec.StandaloneTestStrategy.beginTestAttempt(StandaloneTestStrategy.java:331)
at com.google.devtools.build.lib.exec.StandaloneTestStrategy$StandaloneTestRunnerSpawn.execute(StandaloneTestStrategy.java:582)
at com.google.devtools.build.lib.analysis.test.TestRunnerAction.executeAllAttempts(TestRunnerAction.java:1247)
at com.google.devtools.build.lib.analysis.test.TestRunnerAction.execute(TestRunnerAction.java:1057)
at com.google.devtools.build.lib.analysis.test.TestRunnerAction.execute(TestRunnerAction.java:1019)
at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor$ActionRunner.executeAction(SkyframeActionExecutor.java:1287)
at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor$ActionRunner.run(SkyframeActionExecutor.java:1189)
at com.google.devtools.build.lib.skyframe.ActionExecutionState.runStateMachine(ActionExecutionState.java:166)
at com.google.devtools.build.lib.skyframe.ActionExecutionState.getResultOrDependOnFuture(ActionExecutionState.java:95)
at com.google.devtools.build.lib.skyframe.SkyframeActionExecutor.executeAction(SkyframeActionExecutor.java:690)
at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.checkCacheAndExecuteIfNeeded(ActionExecutionFunction.java:832)
at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.computeInternal(ActionExecutionFunction.java:355)
at com.google.devtools.build.lib.skyframe.ActionExecutionFunction.compute(ActionExecutionFunction.java:207)
at com.google.devtools.build.skyframe.AbstractParallelEvaluator$Evaluate.run(AbstractParallelEvaluator.java:471)
... 8 more
```

### Which category does this issue belong to?

Local Execution

### What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

Crashes on aac8677f8e69e30fb105026ff524e04965640499, passes on 1f05f210b6e7198e4144640bd8044ef7eeaddccd while running `bazel test //... --config=ci` on https://github.com/fmeum/with_cfg.bzl/commit/a33b1b2d2fd38f5884643d37d1c54c5d280d583c.

### Which operating system are you running Bazel on?

Ubuntu

### What is the output of `bazel info release`?

_No response_

### If `bazel info release` returns `development version` or `(@non-git)`, tell us how you built Bazel.

_No response_

### What's the output of `git remote get-url origin; git rev-parse HEAD` ?

```text

```

### If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

_No response_

### Have you found anything relevant by searching the web?

_No response_

### Any other information, logs, or outputs that you want to share?

_No response_

Reproduce the crash with `bazel test //... --config=ci` using the referenced with_cfg.bzl commit, then read `SandboxStash.java`, especially `takeStashedSandboxInternal`, and the callers in `SymlinkedSandboxedSpawn.java`. Done means the Linux test completes without the reported NullPointerException or Bazel crash.

Written by the indexing model from the issue text.