GH Action: merge staging(-next) periodically

Automate the merging of `master` -> `staging-next` -> `staging`.

Our main development branch is `master`. Large rebuilds go to `staging`.
Periodically, `staging` is merged into `staging-next` for stabilization.
When considered sufficiently stable, `staging-next` is merged into
`master`.

As changes arrive on these branches, it is important that they're all
updated regularly with eachothers changes. This commit automates that
part.

.github/CODEOWNERS

@@ -12,6 +12,7 @@
 
 # GitHub actions
 /.github/workflows @Mic92 @zowoq
+/.github/workflows/merge-staging @FRidh
 
 # EditorConfig
 /.editorconfig @Mic92 @zowoq

.github/workflows/merge-staging.yml

@@ -0,0 +1,30 @@
+name: "merge staging(-next)"
+
+on:
+  schedule:
+    # * is a special character in YAML so you have to quote this string
+    # Merge every 6 hours
+    - cron:  '* */6 * * *'
+
+jobs:
+  sync-branch:
+    if: github.repository == 'NixOS/nixpkgs'
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Merge master into staging-next
+        uses: devmasx/merge-branch@v1.3.1
+        with:
+          type: now
+          from_branch: master
+          target_branch: staging-next
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Merge staging-next into staging
+        uses: devmasx/merge-branch@v1.3.1
+        with:
+          type: now
+          from_branch: staging-next
+          target_branch: staging
+          github_token: ${{ secrets.GITHUB_TOKEN }}