fixup: use cache@v4

Author: frasertweedale

.github/workflows/haskell-ci.yml

@@ -1230,7 +1230,7 @@ jobs:
           HCVER: ${{ matrix.compilerVersion }}
       - name: cache /tmp/libnotmuch
         id: cache-notmuch
-        uses: actions/cache@v2
+        uses: actions/cache@v4
         with:
           key: ${{ runner.os }}-notmuch-${{ matrix.notmuch }}
           path: /tmp/notmuch