Fix choice of conda package for includes (on Windows)

The path to CUDA includes may exist in a conda environment even if they
are not present, particularly on Windows where the include path is just
`$CONDA_PREFIX\Library\include`. Therefore, the presence of the include
dir alone doesn't signify that the CUDA includes are actually present in
that location - we also need to check whether a relevant header file
exists.

numba_cuda/numba/cuda/cuda_paths.py

@@ -310,7 +310,9 @@ def get_conda_include_dir():
         # though usually it shouldn't.
         include_dir = os.path.join(sys.prefix, 'include')
 
-    if os.path.exists(include_dir):
+    if (os.path.exists(include_dir) and os.path.isdir(include_dir)
+            and os.path.exists(os.path.join(include_dir,
+                                            'cuda_device_runtime_api.h'))):
         return include_dir
     return