drop github update-flake-lock in favour of renovate bot

.github/workflows/update-flake-lock.yml

@@ -1,25 +0,0 @@
-name: update-flake-lock
-on:
-  workflow_dispatch: # allows manual triggering
-  schedule:
-    - cron: '0 0 * * 1,4' # Run twice a week
-
-jobs:
-  lockfile:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v3
-      - name: Install Nix
-        uses: cachix/install-nix-action@v20
-      - name: Update flake.lock
-        uses: DeterminateSystems/update-flake-lock@v19
-        with:
-          # TODO
-          #token: ${{ secrets.GH_TOKEN_FOR_UPDATES }}
-          pr-body: |
-            Automated changes by the update-flake-lock
-            ```
-            {{ env.GIT_COMMIT_MESSAGE }}
-            ```
-            bors merge