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/ci.yml

@@ -1,17 +0,0 @@
-name: CI
-on:
-  pull_request:
-  push:
-    branches: [main]
-
-jobs:
-  shellcheck:
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v2
-        with:
-          fetch-depth: 0
-      - name: Install Nix
-        uses: cachix/install-nix-action@v16
-      - name: Shellcheck
-        run: nix-shell --run 'shellcheck $(find . -type f -name "*.sh" -executable)'