Clean up some path handling in the schemastore script

[zanieb]

Mostly making sure that the script is robust to alternative working directories, and some stylistic nits.

Following up on #65 (comment) — 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