Improve ci on fix-whitespace installation

Author: liesnikov

.github/workflows/ci.yml

@@ -37,16 +37,19 @@ jobs:
         ghc-version: ${{ matrix.ghc }}
         cabal-version: ${{ matrix.cabal }}
 
-    # Install fix-whitespace since we need it for tests
-    - name: Install whitespace
-      run: cabal install fix-whitespace
-
     # Generate a cabal.project.freeze file with all dependencies. We use the
     # hash of this as the cache key, so when a dependency changes we upload a
     # new cache.
     - name: Freeze
       run: cabal freeze
 
+    # Install fix-whitespace since we need it for tests.
+    # This is done after the freeze to make sure we re-use the packages
+    - name: Install fix-whitespace
+      run: |
+        echo "import: cabal.project" > cabal.project.ci
+        cabal install fix-whitespace --project-file=cabal.project.ci
+
     # Cache the contents of ~/.cabal/store to avoid rebuilding dependencies for
     # every build. `restore-keys` makes it use the latest cache even if the
     # fingerprint doesn't match, so we don't need to start from scratch every