ci(nix): add paths filter to pull_request trigger

- Add paths filter to pull_request trigger so flake-check only runs
  when nix-related files are changed
- Fix workflow file extension from .yml to .yaml for consistency

This reduces unnecessary CI runs on PRs that don't modify nix files.

Author: ryoppippi

.github/workflows/nix-flake.yaml

@@ -5,9 +5,14 @@ on:
     paths:
       - 'flake.nix'
       - 'flake.lock'
-      - '.github/workflows/nix-flake.yml'
+      - '.github/workflows/nix-flake.yaml'
       - '.github/actions/setup-nix/**'
   pull_request:
+    paths:
+      - 'flake.nix'
+      - 'flake.lock'
+      - '.github/workflows/nix-flake.yaml'
+      - '.github/actions/setup-nix/**'
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}