[SPARK-19550][HOTFIX][BUILD] Use JAVA_HOME/bin/java if JAVA_HOME is set in dev/mima

## What changes were proposed in this pull request?

Use JAVA_HOME/bin/java if JAVA_HOME is set in dev/mima script to run MiMa
This follows on https://github.com/apache/spark/pull/16871 -- it's a slightly separate issue, but, is currently causing a build failure.

## How was this patch tested?

Manually tested.

Author: Sean Owen <sowen@cloudera.com>

Closes #16957 from srowen/SPARK-19550.2.

dev/mima

@@ -30,7 +30,13 @@ OLD_DEPS_CLASSPATH="$(build/sbt -DcopyDependencies=false $SPARK_PROFILES "export
 
 rm -f .generated-mima*
 
-java \
+if [[ -x "$JAVA_HOME/bin/java" ]]; then
+  JAVA_CMD="$JAVA_HOME/bin/java"
+else
+  JAVA_CMD=java
+fi
+
+$JAVA_CMD \
   -Xmx2g \
   -cp "$TOOLS_CLASSPATH:$OLD_DEPS_CLASSPATH" \
   org.apache.spark.tools.GenerateMIMAIgnore