Rebase develop after a master push (qmk#10812)

* rebase develop after a master push

* limit this to qmk/qmk_firmware

* change to a merge strategy

* GITHUB_TOKEN isn't needed

* check for .locked in develop

* break up into more steps

.github/workflows/develop_update.yml

@@ -0,0 +1,32 @@
+name: Update develop after master merge
+
+on:
+  push:
+    branches:
+    - master
+
+
+jobs:
+  develop_update:
+    runs-on: ubuntu-latest
+
+    if: github.repository == 'qmk/qmk_firmware'
+
+    steps:
+    - uses: actions/checkout@v2
+      with:
+        fetch-depth: 0
+
+    - name: Checkout develop
+      run: |
+        git fetch origin master develop
+        git checkout develop
+
+    - name: Check if branch locked
+      run: |
+        test -e .locked || exit 1
+
+    - name: Update develop from master
+      run: |
+        git merge origin/master
+        git push origin develop