Flaky has_local in rustup-init.sh

Verification

• I searched for recent similar issues at https://github.com/rust-lang/rustup/issues?q=is%3Aissue and found no duplicates.
• I am on the latest version of Rustup according to https://github.com/rust-lang/rustup/tags and am still able to reproduce my issue.

Problem

On a system without local, it would continue searching in $PATH. If a such a command were found, that would wrongly be used throughout.

Steps

1. Login to where /bin/sh has no builtin local
2. Create an executable local in $PATH

Possible Solution(s)

I don’t know how portable an empty path is, nor whether prepending it will affect searching for the following command on such a Shell. This should probably work anywhere:

has_local() {
    # shellcheck disable=SC2034  # deliberately unused
    PATH= local _has_local
}

Notes

No response

Rustup version

Reading latest script on GitHub.

Installed toolchains

Irrelevant, since I don’t have a Shell with no builtin `local`

OS version

Irrelevant, since I don’t have a Shell with no builtin `local`

Open rustup-init.sh and locate has_local; first reproduce the case with a shell lacking builtin local and an executable named local on PATH. Verify the fix makes has_local test the shell builtin rather than the PATH command, then run the relevant shell checks if available.