Added accidentally-removed cpp-mgtests target back to the valid args list

[rlratzel]

Added accidentally-removed cpp-mgtests target back to the valid args list. This was removed during an update to add --buildfaiss, likely due to a bad merge?

Tested by running with the target and observing the MG tests being built and the MPI library being pulled in (still building, will update if something goes wrong)

[codecov-commenter]

Codecov Report

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@@               Coverage Diff               @@
##             branch-21.08    #1652   +/-   ##
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  Coverage                ?   59.79%           
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  Files                   ?       80           
  Lines                   ?     3542           
  Branches                ?        0           
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  Hits                    ?     2118           
  Misses                  ?     1424           
  Partials                ?        0           

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[ChuckHastings]

rerun tests

[BradReesWork]

@gpucibot merge