workflows: teach dependabot workflow to rebase

Our dependabot workflow is unable to deal with conflicting node_modules
when we land one of the dependabot pull requests. The manual workflow
before was to let dependabot re-create the pull request and then add the
`node_modules` label after re-recreating the pull request. Automating
this is tricky as a Github workflow cannot automate the recreate comment
as it has no write permissions to our projects.

The new approach when adding a `node_modules` label will always rebase
the pull request and in case of a conflict will remove the conflicting
node_modules changes in the commit. Afterwards we create the new
node_modules commit which can be merged.

.github/workflows/dependabot.yml

@@ -41,6 +41,18 @@ jobs:
               }
             }
 
+      - name: Rebase pull request and drop node_modules changes
+        run: |
+          set -x
+          git config --global user.name "GitHub Workflow"
+          git config --global user.email "[email protected]"
+          git fetch origin main:main
+          if ! git rebase main; then
+            git reset node_modules
+            GIT_EDITOR=true git rebase --continue
+            git show --stat
+          fi
+
       - name: Update node_modules for package.json changes
         run: |
           make tools/node-modules