libtorch-bin: fix false CUDA dependency in passthru test

The passthru test used the CUDA toolkit for the CUDA stub
unconditionally, also requiring the cudatoolkit derivation when
cudaSupport = false.

pkgs/development/libraries/science/math/libtorch/test/default.nix

···
     touch $out
   '';
 
-  checkPhase = ''
+  checkPhase = lib.optionalString cudaSupport ''
     LD_LIBRARY_PATH=${cudaStub}''${LD_LIBRARY_PATH:+:}$LD_LIBRARY_PATH \
-      ./test
+  '' + ''
+    ./test
   '';
 }