ci: isolate pnpm store per runner to prevent concurrent corruption

Self-hosted runners share the same home directory. When multiple runners
run concurrently, pnpm/action-setup's shared store at ~/setup-pnpm gets
corrupted (ENOTEMPTY errors, missing files). Fix: configure a per-runner
store-dir before install so each runner gets its own isolated pnpm store.

.github/workflows/ci.yml

@@ -35,6 +35,9 @@ jobs:
       - name: Install cargo-llvm-cov
         uses: taiki-e/install-action@cargo-llvm-cov
 
+      - name: Configure pnpm store (per-runner isolation)
+        run: pnpm config set store-dir "$HOME/.pnpm-store-${{ runner.name }}"
+
       - name: Install dependencies
         run: pnpm install --frozen-lockfile