Mostly making sure that the script is robust to alternative working
directories, and some stylistic nits.
Following up on
https://github.com/astral-sh/ty/pull/65#discussion_r2077839514 — not to
be annoying, but using `git` to find the root _can_ be wrong here
```
❯ cd ruff
❯ uv run --only-dev ../scripts/update_schemastore.py
> /Users/zb/workspace/ty/scripts/update_schemastore.py(146)main()
-> breakpoint()
(Pdb) print(root)
/Users/zb/workspace/ty/ruff
```
