fix: replace arch with uname -m

[bas-ie]

This:

• replaces arch with uname -m

Why? arch is no longer included in coreutils by default. This will probably cause some confusion for users of up-to-date distributions, who will see something like

arch: command not found

when trying to use this plugin. uname -m does the same thing, so may as well use that 🙂

Thanks for the plugin!

[wbh1]

This has the additional benefit of fixing broken installations on Intel Macs.

Before I would get 404's:

asdf-ruff: Could not download https://github.com/astral-sh/ruff/releases//download/0.6.9/ruff-i386-apple-darwin.tar.gz

With @richchurcher 's branch, I can successfully install ruff.