Allow running previously downloaded installer after failed rustup

[jdm]

I've been having connection timeouts while using rustup all afternoon today, and each time I restart it it has to download the installer all over again. It would be nice if rustup were smart enough to cache artifacts and avoid downloading them again.

[canfoderiskii]

Yeah, it would be perfect if this feature is added. I often encounter the timeout problem. It's quite annoying.

[Diggsey]

Should have been fixed in #958