Fix GitHub Actions: Stop caching ~/.cabal/packages directory.

~/.cabal/packages stores hackage tarballs and those archive is not
needed for already built packages, so it is sufficient to cache ~/.cabal/store.

.github/workflows/haskell.yml

@@ -24,13 +24,6 @@ jobs:
         ghc-version: ${{ matrix.ghc }}
         cabal-version: ${{ matrix.cabal }}
 
-    - name: Cache ~/.cabal/packages
-      uses: actions/cache@v1
-      with:
-        path: ~/.cabal/packages
-        key: ${{ runner.os }}-cabal-packages-cabal${{ matrix.cabal }}-ghc${{ matrix.ghc }}
-        restore-keys: |
-          ${{ runner.os }}-cabal-packages-cabal${{ matrix.cabal }}-
     - name: Cache ~/.cabal/store
       uses: actions/cache@v1
       with:
@@ -40,7 +33,7 @@ jobs:
     - name: Install dependencies
       run: |
         cabal v2-update
-        cabal v2-build all --only-dependencies
+        cabal v2-build all --only-dependencies --enable-tests
     - name: Build
       run: cabal v2-build all
     - name: Run tests