Don't use an external script for multiple inputs

All consumers would then have to bring this script into their repo,
since GitHub doesn't have something like Nix's string context.

.github/workflows/update.yml

@@ -1,19 +0,0 @@
-name: update-flake-lock
-on:
-  workflow_dispatch:
-  schedule:
-    - cron: '0 0 * * 0'
-
-jobs:
-  lockfile:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v2
-      - name: Install Nix
-        uses: cachix/install-nix-action@v16
-        with:
-          extra_nix_config: |
-            access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
-      - name: Update flake.lock
-        uses: ./.