workflows: build os on changes

.github/workflows/os.yml

@@ -0,0 +1,18 @@
+name: os
+on:
+  push:
+    paths:
+    - "os/**"
+  pull_request:
+    paths:
+    - "os/**"
+jobs:
+  build:
+    name: build
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - run: ./hack/ci/install-deps.sh
+    - run: ./hack/os/build.sh
+      env:
+        KRATA_KERNEL_BUILD_JOBS: "5"