[Build] Fixed generate_gmake2_vulkan.py

The script was previously passing the d3d12 parameter to the project
generator instead of vulkan.

Apart from that, fixed an issue where subprocess.Popen didn't actually
work. Aparently adding `shell=True` as a parameter fixes it.

generate_gmake2_vulkan.py

@@ -4,6 +4,6 @@
 # change working directory to script directory
 os.chdir(os.path.dirname(__file__))
 # run script
-subprocess.Popen("python build_scripts\\generate_project_files.py gmake2 d3d12").communicate()
+subprocess.Popen("python3 build_scripts/generate_project_files.py gmake2 vulkan", shell=True).communicate()
 # exit
 sys.exit(0)