CI: add actionlint

.github/workflows/actionlint-exceptions.txt

@@ -0,0 +1,4 @@
+.github/workflows/haskell.yml
+.github/workflows/release-ghcr.yaml
+.github/workflows/release-upload.yaml
+.github/workflows/stylish-haskell.yml

.github/workflows/actionlint.yml

@@ -0,0 +1,44 @@
+name: Actionlint
+
+on:
+  pull_request:
+
+jobs:
+  actionlint:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      # We want to install Nix to provision shellcheck, so that actionlint doesn't install
+      # its own shellcheck. This will also make sure that this pipeline runs using
+      # the same shellcheck as the ones in Nix shells of developers.
+      - name: Install Nix with good defaults
+        uses: input-output-hk/install-nix-action@v20
+        with:
+          extra_nix_config: |
+            trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ=
+            substituters = https://cache.iog.io/ https://cache.nixos.org/
+          nix_path: nixpkgs=channel:nixos-unstable
+      - uses: cachix/install-nix-action@v18
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      # Make the Nix environment available to next steps
+      - uses: rrbutani/use-nix-shell-action@v1
+
+      - name: Install actionlint
+        run: |
+          # Puts the "actionlint" binary in the working folder
+          bash <(curl https://raw.githubusercontent.com/rhysd/actionlint/main/scripts/download-actionlint.bash)
+
+      - name: Run Actionlint
+        run: |
+          for file in $(git ls-files ".github/workflows/*.y*ml")
+          do
+            if grep -q "$file" ".github/workflows/actionlint-exceptions.txt"
+            then
+              echo "⚠️ $file is ignored from actionlint's verifications. Please consider fixing it."
+            else
+              echo "./actionlint $file"
+              ./actionlint "$file"
+            fi
+          done