Fix has_local to not search PATH for external 'local' command

[harriiinnii]

Problem

has_local() in rustup-init.sh is used to detect whether the shell has a builtin local keyword. On shells that lack it, the function falls through and the script aliases local to typeset.

However, if the shell has no builtin local and an executable named local happens to exist somewhere in $PATH, that external command would be found and used instead — making has_local return success incorrectly. Any subsequent local usage would then invoke that external command rather than the typeset alias.

Fix

Set PATH= before the local call inside has_local. This prevents the shell from searching $PATH for a local executable: on shells with a builtin local the call succeeds as before, and on shells without it the call correctly fails even if an external local exists in $PATH.

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

Fixes #5009

[rami3l]

@harriiinnii Oops, it seems like your change is not a pure refactoring step? Would you mind doing some more digging?