Only add the actual schema in schemastore PRs (#20947)

Same as https://github.com/astral-sh/ty/pull/1391:

> Last time I ran this script, due to what I assume was a `npm` version
mismatch, the `package-lock.json` file was updated while running `npm
install` in the `schemastore`. Due to the use of `git commit -a`, it was
accidentally included in the commit for the semi-automated schemastore
PR. The solution here is to only add the actual file that we want to
commit.

scripts/update_schemastore.py

@@ -117,11 +117,11 @@ def update_schemastore(
             f"This updates ruff's JSON schema to [{current_sha}]({commit_url})"
         )
         # https://stackoverflow.com/a/22909204/3549270
+        check_call(["git", "add", (src / RUFF_JSON).as_posix()], cwd=schemastore_path)
         check_call(
             [
                 "git",
                 "commit",
-                "-a",
                 "-m",
                 "Update ruff's JSON schema",
                 "-m",