[fix](java) export JAVA_OPTS at last

[morningman]

What problem does this PR solve?

Related PR: #47913

Problem Summary:

Export the JAVA_OPTS at last in start_xxx.sh.
Because user may use different version of java to run Doris, so different JAVA_OPTS
env variable may be used like JAVA_OPTS for java8, JAVA_OPTS_FOR_JDK_17 for java 17.

But in be/src/util/jni-util.cpp, it only read JAVA_OPTS env variable.
So we need to set this variable at last.

[Thearas]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[morningman]

run buildall

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 36.50% (9610/26327)
Line Coverage: 28.10% (79680/283609)
Region Coverage: 26.75% (40861/152778)
Branch Coverage: 23.51% (20716/88126)
Coverage Report: http://coverage.selectdb-in.cc/coverage/92b3d0c36c32886649b6734f99e827609c094856_92b3d0c36c32886649b6734f99e827609c094856/report/index.html