introduce fourmolu to ci

Author: MaximilianAlgehed

.github/workflows/haskell-ci.yml

@@ -23,6 +23,18 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  check-formatting:
+    name: Check formatting
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: install fourmolu
+        run: |
+          wget https://github.com/fourmolu/fourmolu/releases/download/v0.17.0.0/fourmolu-0.17.0.0-linux-x86_64
+          chmod +x fourmolu-0.17.0.0-linux-x86_64
+          mv fourmolu-0.17.0.0-linux-x86_64 fourmolu
+      - run: ./fourmolu -c .
+
   test-with-cabal:
     name: Haskell-CI - Linux - ${{ matrix.ghc-version }}