[SPARK-36007][INFRA] Failed to run benchmark in GA

### What changes were proposed in this pull request?

When I'm running the benchmark in GA, I met the below error.

https://github.com/pingsutw/spark/runs/2867617238?check_suite_focus=true
```
java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.j
ava:1692)java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:175)
21/06/20 07:40:02 ERROR SparkContext: Error initializing SparkContext.java.lang.AssertionError: assertion failed:
spark.test.home is not set! at scala.Predef$.assert(Predef.scala:223) at org.apache.spark.deploy.worker.Worker.<init>
(Worker.scala:148) at org.apache.spark.deploy.worker.Worker$.startRpcEnvAndEndpoint(Worker.scala:954) at
org.apache.spark.deploy.LocalSparkCluster.$anonfun$start$2(LocalSparkCluster.scala:68) at
org.apache.spark.deploy.LocalSparkCluster.$anonfun$start$2$adapted(LocalSparkCluster.scala:65) at
scala.collection.immutable.Range.foreach(Range.scala:158) at
org.apache.spark.deploy.LocalSparkCluster.start(LocalSparkCluster.scala:65) at
org.apache.spark.SparkContext$.org$apache$spark$SparkContext$$createTaskScheduler(SparkContext.scala:2954) at
org.apache.spark.SparkContext.<init>(SparkContext.scala:559) at org.apache.spark.SparkContext.<init>
(SparkContext.scala:137) at
org.apache.spark.serializer.KryoSerializerBenchmark$.createSparkContext(KryoSerializerBenchmark.scala:86) at
org.apache.spark.serializer.KryoSerializerBenchmark$.sc$lzycompute$1(KryoSerializerBenchmark.scala:58) at
org.apache.spark.serializer.KryoSerializerBenchmark$.sc$1(KryoSerializerBenchmark.scala:58) at
org.apache.spark.serializer.KryoSerializerBenchmark$.$anonfun$run$3(KryoSerializerBenchmark.scala:63)
```

### Why are the changes needed?

Set `spark.test.home` in the benchmark workflow.

### Does this PR introduce _any_ user-facing change?

No

### How was this patch tested?

Rerun the benchmark in my fork.
https://github.com/pingsutw/spark/actions/runs/996067851

Closes #33203 from pingsutw/SPARK-36007.

Lead-authored-by: Kevin Su <pingsutw@apache.org>
Co-authored-by: Kevin Su <pingsutw@gmail.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

.github/workflows/benchmark.yml

@@ -48,6 +48,8 @@ jobs:
       SPARK_BENCHMARK_CUR_SPLIT: ${{ matrix.split }}
       SPARK_GENERATE_BENCHMARK_FILES: 1
       SPARK_LOCAL_IP: localhost
+      # To prevent spark.test.home not being set. See more detail in SPARK-36007.
+      SPARK_HOME: ${{ github.workspace }}
     steps:
     - name: Checkout Spark repository
       uses: actions/checkout@v2