mesa: correctly use os.path.join in our python scripts

mesa: correctly use os.path.join in our python scripts

With Windows in mind, using forward slash isn't the right thing to do.
Even if it just works, we might want to fix it.

As here, use __file__ instead of argv[0] and sys.path.insert over
sys.path.append. With the path tweak being reportedly faster.

Suggested-by: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>