explicit sync action input on branch

otherwise github requires the {OS}-one branches to contain
the github workflow as well (which we preferably avoid).

.github/workflows/sync-unstable.yaml

@@ -2,22 +2,22 @@ name: sync-unstable
 
 on:
   workflow_dispatch:
+    inputs:
+      branch:
+        description: 'branch to sync to (no -unstable suffix)'
+        required: true
 
 jobs:
   sync:
     runs-on: ubuntu-22.04
     steps:
       - name: checkout unstable branch
         uses: actions/checkout@v4
-      - name: validate branch
-        run: |
-          if [[ ${{ github.event.inputs.repo }} != *-unstable ]]; then
-            echo "This workflow is only meant for -unstable branches"
-            exit 1
-          fi
+        with:
+          ref: ${{ github.event.inputs.branch }}-unstable
+          fetch-depth: 0
       - name: Strip -unstable suffix
         run: |
-          echo "BRANCH=$(echo ${{ github.event.inputs.repo }} | sed 's/-unstable$//')" >> $GITHUB_ENV
           find . -type f -exec sed -i 's/-unstable//g' {} +
       - name: Add sync date to README
         run: |
@@ -26,7 +26,7 @@ jobs:
         uses: s0/git-publish-subdir-action@develop
         env:
           REPO: self
-          BRANCH: ${{ env.BRANCH }}
+          BRANCH: ${{ github.event.inputs.branch }}
           FOLDER: .
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
           SQUASH_HISTORY: true