chore: add script to update flake automatically (#14046)

.github/workflows/ci.yaml

@@ -121,6 +121,9 @@ jobs:
     needs: changes
     if: needs.changes.outputs.gomod == 'true'
     runs-on: ${{ github.repository_owner == 'coder' && 'depot-ubuntu-22.04-8' || 'ubuntu-latest' }}
+    permissions:
+      # Give the default GITHUB_TOKEN write permission to commit and push the changed files back to the repository.
+      contents: write
     steps:
       - name: Checkout
         uses: actions/checkout@v4
@@ -133,8 +136,10 @@ jobs:
       - name: Update Nix Flake SRI Hash
         run: ./scripts/update-flake.sh
 
-      - name: Ensure No Changes
-        run: git diff --exit-code
+      - uses: stefanzweifel/git-auto-commit-action@v5
+        with:
+          # Allows dependabot to still rebase!
+          commit_message: "[dependabot skip] Update Nix Flake SRI Hash"
 
   lint:
     needs: changes