Add TACO_NVCC var to complement TACO_NVCCFLAGS

This is useful for passing specific arguments to nvcc.  In my case,
I wanted to force nvcc to use a specific version of g++.

src/codegen/module.cpp

@@ -122,7 +122,7 @@ string Module::compile() {
   string file_ending;
   string shims_file;
   if (should_use_CUDA_codegen()) {
-    cc = "nvcc";
+    cc = util::getFromEnv("TACO_NVCC", "nvcc");
     cflags = util::getFromEnv("TACO_NVCCFLAGS",
     get_default_CUDA_compiler_flags());
     file_ending = ".cu";