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ARROW-7393: [Plasma] Fix plasma executable name in plasma_java build

# What does this PR do? (please provide any background)
It fixes var naming into the arrow/java/plasma/test.sh file that prevented java from finding the built binaries

Closes #6015 from DonaCrio/master and squashes the following commits:

a9e349fd5 <donacrio> fix for linter
62e7fc57a <Donatien Criaud> Merge branch 'master' into master
db7f4be01 <donacrio> fix test.sh for java plasma install
714e5fc6b <donacrio> remove flatbuffers dep

Lead-authored-by: Donatien Criaud <30463908+DonaCrio@users.noreply.github.com>
Co-authored-by: donacrio <donatien.criaud@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>