[SPARK-3734] DriverRunner should not read SPARK_HOME from submitter's environment

When using spark-submit in `cluster` mode to submit a job to a Spark Standalone
cluster, if the JAVA_HOME environment variable was set on the submitting
machine then DriverRunner would attempt to use the submitter's JAVA_HOME to
launch the driver process (instead of the worker's JAVA_HOME), causing the
driver to fail unless the submitter and worker had the same Java location.

This commit fixes this by reading JAVA_HOME from sys.env instead of
command.environment.

Author: Josh Rosen <joshrosen@apache.org>

Closes #2586 from JoshRosen/SPARK-3734 and squashes the following commits:

e9513d9 [Josh Rosen] [SPARK-3734] DriverRunner should not read SPARK_HOME from submitter's environment.

core/src/main/scala/org/apache/spark/deploy/worker/CommandUtils.scala

@@ -30,7 +30,7 @@ import org.apache.spark.util.Utils
 private[spark]
 object CommandUtils extends Logging {
   def buildCommandSeq(command: Command, memory: Int, sparkHome: String): Seq[String] = {
-    val runner = getEnv("JAVA_HOME", command).map(_ + "/bin/java").getOrElse("java")
+    val runner = sys.env.get("JAVA_HOME").map(_ + "/bin/java").getOrElse("java")
 
     // SPARK-698: do not call the run.cmd script, as process.destroy()
     // fails to kill a process tree on Windows
@@ -38,9 +38,6 @@ object CommandUtils extends Logging {
       command.arguments
   }
 
-  private def getEnv(key: String, command: Command): Option[String] =
-    command.environment.get(key).orElse(Option(System.getenv(key)))
-
   /**
    * Attention: this must always be aligned with the environment variables in the run scripts and
    * the way the JAVA_OPTS are assembled there.