Installation fails on aarch64

[andriygm]

Describe the bug

uv stopped distributing the aarch64-unknown-linux-gnu binary (see astral-uv/sh #3761), but this plugin has a hardcoded dependency for unknown-linux-gnu if the distro is Linux:

asdf-uv/bin/download

Line 22 in f339c19

Linux) os="unknown-linux-gnu" ;;

This should instead resolve based on the architecture instead of hardcoding.

Steps to reproduce

❯ asdf plugin add uv
initializing plugin repository...Cloning into '/home/***/.asdf/repository'...
remote: Enumerating objects: 5854, done.
remote: Counting objects: 100% (1639/1639), done.
remote: Compressing objects: 100% (63/63), done.
remote: Total 5854 (delta 1590), reused 1611 (delta 1575), pack-reused 4215
Receiving objects: 100% (5854/5854), 1.32 MiB | 5.75 MiB/s, done.
Resolving deltas: 100% (3202/3202), done.
❯ asdf install uv latest
redirect url: https://github.com/astral-sh/uv/releases/tag/0.2.4
* Downloading uv release 0.2.4...
curl: (22) The requested URL returned error: 404
asdf-uv: Could not download https://github.com/astral-sh/uv/releases/download/0.2.4/uv-aarch64-unknown-linux-gnu.tar.gz

Expected behavior

On aarch64 systems, the musl binary is used instead.