The linkargs and compileargs sandbox parameters are relevant to C jobs but you're actually running a Python job. Those parameters aren't relevant to Python.
The c_via_python question type is not intended to be the primary way to run a C program. It's an example of how to implement a different language by writing a Python script to compile and run a program in that new language. The commands to compile and build the C program are embedded within the Python script. If you wish to set compile args and link args for this question type you'll have to insert them into the Python script.
Or you can just go back to using one of the built-in C question types like c_function or c_program, as you apparently were doing at the start of this thread.