CI: do cabal update after restoring cache to aviod mixup due to XDG

See haskell/actions#203

.github/workflows/validate.yml

@@ -72,6 +72,7 @@ jobs:
         with:
           ghc-version: ${{ matrix.ghc }}
           cabal-version: '3.8.1.0'
+          cabal-update: false
 
       #  See the following link for a breakdown of the following step
       #  https://github.com/haskell/actions/issues/7#issuecomment-745697160
@@ -87,6 +88,10 @@ jobs:
           key: ${{ runner.os }}-${{ matrix.ghc }}-20220419-${{ github.sha }}
           restore-keys: ${{ runner.os }}-${{ matrix.ghc }}-20220419-
 
+      - name: cabal update due to https://github.com/haskell/actions/issues/203
+        run: |
+          cabal update
+
       - name: Work around git problem https://bugs.launchpad.net/ubuntu/+source/git/+bug/1993586 (cabal PR #8546)
         run: |
           git config --global protocol.file.allow always