make it possible to add args to cargo in x.py
make it possible to add args to cargo in x.py

eg. make it easier to test -Ztimings for rustc

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust