Convert posix scripts to bash

[Daniel-Worrall]

Addresses rust-lang/rust#31036

E: Force was because I saw the file had CRLF, not LF, so fixed.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[alexcrichton]

Sorry but I don't have time to review this, I'd recommend working with folks from rust-lang/rust to review this.

[Mark-Simulacrum]

r? @Mark-Simulacrum for now, though it's unlikely that we'll review this before the main rustc PR has a decision made it on it

[Daniel-Worrall]

Closing after it was assessed by the infra team. rust-lang/rust#71932 (comment)