cuda: Create obj dir in generator script.

This adds creation of the obj/ dir, which is needed to generate the
version of the Blake3 kernel used in CUDA builds.

The Windows version no longer needs the obj/ dir, therefore that is not
necessary on that platform.

cuda_builder.go

@@ -201,7 +201,8 @@ func checkRequirementsWindows() error {
 // checkRequirementsDefault checks the requirements for any other OS/architecture
 // combinations.
 func checkRequirementsDefault() error {
-	return nil
+	// Create the obj/ dir.
+	return os.MkdirAll("obj", 0o755)
 }
 
 // buildBlake3Windows builds the blake3.dll library with a compiled version of