Avoid rebuilding cython in build.sh

[brandon-b-miller]

Fixes an issue where python setup.py install is called after build_ext already finishes running causing all the cython files to be recompiled one by one.

[codecov]

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@@               Coverage Diff               @@
##             branch-21.10    #9034   +/-   ##
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  Coverage                ?   10.65%           
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  Files                   ?      116           
  Lines                   ?    18676           
  Branches                ?        0           
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  Hits                    ?     1990           
  Misses                  ?    16686           
  Partials                ?        0           

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[brandon-b-miller]

@gpucibot merge