rustup-init should detect early if /proc isn't mounted

[joshtriplett]

Running rustup-init.sh without /proc mounted produces many errors before finally explaining the issue:

head: cannot open '/proc/self/exe' for reading: No such file or directory
rustup: unknown platform bitness
./rustup-init.sh: 338: [: Illegal number: 
info: downloading installer
error: Unable to proceed. Could not locate working directory.
error: caused by: no /proc/self/exe available. Is /proc mounted?

All of these ultimately come from the same cause. Rather than proceeding and encountering issues like [: Illegal number, I think it would make sense to detect early in the script if /proc isn't mounted (such as with test -L /proc/self/exe) and error out at that point.

[kinnison]

I got a little overexcited with Robert's approval of the PR, but I believe this is fixed and so will be in 1.24's release.