Was testing rustup on a very minimal Debian installation and got errors during the install process (error occurred in `install.sh` of the Rust nightly.)
Noticed that Rustup was downloading the i686 nightly instead of x86-64. Installing `file` fixed the problem, and this patch adds the probe to ensure file is installed before attempting to use it.
There may still be an issue with the i686 installation, I did not investigate further.
This closes#19168. It's possible that if the downloading of `rustup.sh`
is interrupted, bad things could happen, such as running a naked
"rm -rf /" instead of "rm -rf /path/to/tmpdir". This wraps rustup.sh's
functionality in a function that gets called at the last time that should
protect us from these truncation errors.
Using the current directory may not always be appropriate, for example in
the case where it will unnecessarily trigger a backup to be made.
The only risk with this change is that systems might not have a mktemp.
I am not aware of such a system, but have not tested on Windows. It is
working on a basic Ubuntu and OS X installation.
