perf: add path filtering to Nix validation CI job (Fission-AI#518)

Skip Nix flake validation when no Nix-related files change. This speeds
up CI for PRs that only touch source code, docs, or tests.

Files that trigger Nix validation:
- flake.nix, flake.lock
- package.json, pnpm-lock.yaml
- scripts/update-flake.sh
- .github/workflows/ci.yml

Uses dorny/paths-filter@v3 for change detection. Required-checks jobs
updated to handle skipped status correctly.

Author: TabishB

.github/workflows/ci.yml

@@ -15,6 +15,29 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  # Detect which files changed to enable path-based filtering
+  changes:
+    name: Detect changes
+    runs-on: ubuntu-latest
+    outputs:
+      nix: ${{ steps.filter.outputs.nix }}
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Check for Nix-related changes
+        uses: dorny/paths-filter@v3
+        id: filter
+        with:
+          filters: |
+            nix:
+              - 'flake.nix'
+              - 'flake.lock'
+              - 'package.json'
+              - 'pnpm-lock.yaml'
+              - 'scripts/update-flake.sh'
+              - '.github/workflows/ci.yml'
+
   test_pr:
     name: Test
     runs-on: ubuntu-latest
@@ -160,6 +183,8 @@ jobs:
     name: Nix Flake Validation
     runs-on: ubuntu-latest
     timeout-minutes: 10
+    needs: changes
+    if: needs.changes.outputs.nix == 'true'
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
@@ -262,10 +287,14 @@ jobs:
             echo "Lint job failed"
             exit 1
           fi
-          if [[ "${{ needs.nix-flake-validate.result }}" != "success" ]]; then
+          # Nix validation may be skipped if no Nix-related files changed
+          if [[ "${{ needs.nix-flake-validate.result }}" != "success" && "${{ needs.nix-flake-validate.result }}" != "skipped" ]]; then
             echo "Nix flake validation job failed"
             exit 1
           fi
+          if [[ "${{ needs.nix-flake-validate.result }}" == "skipped" ]]; then
+            echo "Nix flake validation skipped (no Nix-related changes)"
+          fi
           echo "All required checks passed!"
 
   required-checks-main:
@@ -284,8 +313,12 @@ jobs:
             echo "Lint job failed"
             exit 1
           fi
-          if [[ "${{ needs.nix-flake-validate.result }}" != "success" ]]; then
+          # Nix validation may be skipped if no Nix-related files changed
+          if [[ "${{ needs.nix-flake-validate.result }}" != "success" && "${{ needs.nix-flake-validate.result }}" != "skipped" ]]; then
             echo "Nix flake validation job failed"
             exit 1
           fi
+          if [[ "${{ needs.nix-flake-validate.result }}" == "skipped" ]]; then
+            echo "Nix flake validation skipped (no Nix-related changes)"
+          fi
           echo "All required checks passed!"