rustbuild: Update Cargo download location
I updated the beta compiler used to bootstrap the master branch in #38438 with
the intention of fixing Travis OSX linkage issues but I mistakenly forgot that
the PR only updated rustc, not Cargo itself. Cargo has a new release process
with downloads in a different location, so this commit updates rustbuild to
download from this new location by tracking revisions instead of Cargo nightly
dates.