Resurrect CI and update actions

.github/workflows/ci.yml

@@ -0,0 +1,17 @@
+name: CI
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  shellcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+        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)'