Import rapids-cmake modules using the correct cmake variable.

[robertmaynard]

Use rapids-cmake-dir when computing the location of rapids-cmake modules.

Corrects build errors that occur when CUDF_BUILD_BENCHMARKS is enabled.

[codecov]

Codecov Report

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@@               Coverage Diff               @@
##             branch-21.10    #9149   +/-   ##
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  Coverage                ?   10.85%           
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  Files                   ?      114           
  Lines                   ?    19068           
  Branches                ?        0           
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  Hits                    ?     2070           
  Misses                  ?    16998           
  Partials                ?        0           

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[harrism]

@gpucibot merge