add shellcheck to flake instead

.github/workflows/test.yml

@@ -1,18 +0,0 @@
-name: test
-on:
-  workflow_dispatch:
-  pull_request:
-    types: [opened, reopened, synchronize]
-
-jobs:
-  lint:
-    runs-on: ubuntu-22.04
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v3.2.0
-      - name: Install Nix
-        uses: cachix/install-nix-action@v18
-        with:
-          extra_nix_config: access-tokens = github.com=${{ secrets.GITHUB_TOKEN }}
-      - name: ShellCheck
-        run: nix-shell --packages git shellcheck --pure --run 'shellcheck disk-deactivate/disk-deactivate disko'