In return_env, avoid checking isfile.

Since `isfile` is already called on the paths processed by VCRuntimeRedist, this call is redundant, but it also fails if the result from VCRuntimeRedist was None. Fixes #1902

setuptools/msvc.py

@@ -1469,7 +1469,7 @@ def return_env(self, exists=True):
                 exists,
             ),
         )
-        if self.vs_ver >= 14 and isfile(self.VCRuntimeRedist):
+        if self.vs_ver >= 14 and self.VCRuntimeRedist:
             env['py_vcruntime_redist'] = self.VCRuntimeRedist
         return env