Fix variable name in Java build script

[jlowe]

Fixes an incorrect variable reference in java/ci/build-in-docker.sh for configuring whether C++ tests should be built.

[codecov]

Codecov Report

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@@               Coverage Diff               @@
##             branch-21.10    #9161   +/-   ##
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  Coverage                ?   10.82%           
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  Files                   ?      115           
  Lines                   ?    19122           
  Branches                ?        0           
===============================================
  Hits                    ?     2070           
  Misses                  ?    17052           
  Partials                ?        0           

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[jlowe]

@gpucibot merge