## 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.