rustup-init.sh: Check for /proc early
rustup-init.sh: Check for /proc early

Check for /proc being mounted as soon as we identify that we're
running the installer on Linux.  We need /proc for various reasons
and it'd be better to error out cleanly than to report spurious
messages and then error out later in a confusing way.

This fixes #2654

Signed-off-by: Daniel Silverstone <3cc17a76d780e0cd3d87bad586a1c53967be5625@digital-scurf.org>

rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs