CI: fix path for cabal global store.

The GitHub action now uses `$XDG_STATE_HOME/cabal`, which is
why our cache wasn't working.

See https://github.com/haskell/actions/issues/210

.github/workflows/ci.yml

@@ -85,7 +85,7 @@ jobs:
       uses: actions/cache@v3
       with:
         path: |
-          ~/.cabal
+          $XDG_STATE_HOME/cabal
         key: ${{ runner.os }}-${{ matrix.versions.ghc }}-${{ matrix.versions.cabal }}-cabal-global-${{ secrets.CACHE_VERSION }}
 
     - name: Cache cabal work
@@ -152,7 +152,7 @@ jobs:
       uses: actions/cache@v3
       with:
         path: |
-          ~/.cabal
+          $XDG_STATE_HOME/cabal
         key: ${{ runner.os }}-nix-cabal-global-${{ secrets.CACHE_VERSION }}
     - name: Cache cabal work
       id:   cabal-local