Detect '-y' short option in rustup-init.sh more robustly

[umanwizard]

Fixes #2813

[kinnison]

Thank you for making this change, before I review it fully, I notice it uses the getopts shell builtin -- are you confident this will work with /bin/sh on BSDs and Illumos (i.e. ksh and friends)? If so then good, but if you can't be sure then we need to find some people to test it because we don't have anything to that effect in our CI currently really.

[umanwizard]

This uses pure POSIX shell features, so it should work on any Unix-like OS.

I have tested on FreeBSD and it works fine there.

I was not able to test the full script on Illumos because the version I tried does not support rustup at all, but I have tested getopts and shell substitution syntax there and verified that they work the way POSIX specifies, so I am confident that the script will work there too.

[kinnison]

Okay thanks for doing that check.

[umanwizard]

Ping, @kinnison , can this be merged?

[kinnison]

Yep, thank you for your contribution.