Update actions/cache requirement to v2.1.4 (#328)

Updates the requirements on [actions/cache](https://github.com/actions/cache) to permit the latest version.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/commits/26968a09c0ea4f3e233fdddbafd1166051a095f6)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/haskell-ci.yml

@@ -119,7 +119,7 @@ jobs:
         run: |
           $CABAL v2-update -v
       - name: cache (tools)
-        uses: actions/cache@v2
+        uses: actions/cache@v2.1.4
         with:
           key: ${{ runner.os }}-${{ matrix.ghc }}-tools-2f29a7e3
           path: ~/.haskell-ci-tools
@@ -173,7 +173,7 @@ jobs:
           $CABAL v2-build $ARG_COMPILER $ARG_TESTS $ARG_BENCH --dry-run all
           cabal-plan
       - name: cache
-        uses: actions/cache@v2
+        uses: actions/cache@v2.1.4
         with:
           key: ${{ runner.os }}-${{ matrix.ghc }}-${{ github.sha }}
           path: ~/.cabal/store