fix: only run rust-ci.yml on PRs that modify files in codex-rs (#637)

The `rust-ci.yml` build appears to be a bit flaky (we're looking into
it...), so to save TypeScript contributors some noise, restrict the
`rust-ci.yml` job so that it only runs on PRs that touch files in
`codex-rs/`.

.github/workflows/rust-ci.yml

@@ -1,7 +1,13 @@
 name: rust-ci
 on:
-  pull_request: { branches: [main] }
-  push: { branches: [main] }
+  pull_request:
+    branches:
+      - main
+    paths:
+      - "codex-rs/**"
+  push:
+    branches:
+      - main
 
 # For CI, we build in debug (`--profile dev`) rather than release mode so we
 # get signal faster.