Update actions/cache action to v2.1.8

Author: renovate[bot]

.github/workflows/release-notes.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Fetch all tags and branches
         run: git fetch --prune --unshallow
       - name: Cache Tools
-        uses: actions/cache@v2.1.4
+        uses: actions/cache@2b250bc32ad02700b996b496c14ac8c2840a2991 # v2.1.8
         with:
           path: tools
           key: ${{ runner.os }}-tools-${{ hashFiles('recipe.cake') }}