jiri: Use runtil.IsNotExist instead of os.IsNotExist.
jiri: Use runtil.IsNotExist instead of os.IsNotExist.

The Sequence helper wraps its errors, so we must use runtil.IsNotExist
to see if the new error is a NotExist error, instead of relying on
on.IsNotExist, which only works for unwrapped errors.

Change-Id: I11941f6307935a5325f9ead8cb98705bfb704ce1

vanadium/go.jiri,vanadium/go.jiri