ci: add kernel-sync check

This check compares torvalds/master `rust/pin-init` with this repo's
`kernel` branch and fails if they are not in sync.

Signed-off-by: Benno Lossin <benno.lossin@proton.me>

Author: BennoLossin

.github/workflows/kernel.yml

@@ -0,0 +1,23 @@
+permissions:
+  contents: read
+on:
+  push:
+    branches: [kernel]
+  schedule:
+    - cron: '7 7 * * *'
+name: kernel
+jobs:
+  sync:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        ref: kernel
+        path: pin-init
+      - uses: actions/checkout@v4
+        repository: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
+        ref: master
+        path: kernel
+        fetch-depth: 1
+      - run: |
+          cp -r kernel/rust/pin-init .
+          git diff --quiet && git diff --cached --quiet && [ -z "$(git ls-files --others --exclude-standard)" ]