.github/workflows/ci.yml: use restore-keys (#1279)

.github/workflows/ci.yml

@@ -50,7 +50,9 @@ jobs:
         uses: actions/cache@v2.1.6
         with:
           path: ~/.stack/snapshots
-          key: ${{ runner.os }}-${{ hashFiles(format('./{0}', matrix.versions.stack-yaml)) }}
+          key: ${{ runner.os }}-${{ matrix.versions.stack-yaml }}-${{ hashFiles(format('./{0}', matrix.versions.stack-yaml)) }}
+          restore-keys: |
+            ${{ runner.os }}-${{ matrix.versions.stack-yaml }}
       - name: Update system dependencies
         run: |
           sudo apt update