Add probe and var for file to rustup.sh

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.