Retry downloads

[gnzlbg]

In libc, some of our build jobs use rustup target add <target> many times, and some times, these build jobs fail spuriously because one of those commands fails to download what's required. Retrying the download solves the issue.

There should be an easy way to get rustup to retry downloading artifacts a couple of times. Pretty much every application that downloads artifacts has this, and it is a pain to work around the lack of this feature in POSIX compliant shell scripts.