fix(ci): enable key caching for DF testing.

.github/workflows/continuous-integration.yml

@@ -99,6 +99,15 @@ jobs:
         with:
           submodules: recursive
 
+      - name: "KeySetCache"
+        uses: actions/cache@v2
+        with:
+          path: ${{ github.workspace }}/KeySetCache
+          # actions/cache does not permit to update a cache entry
+          key: ${{ runner.os }}-KeySetCache-2021-12-02
+          restore-keys: |
+            ${{ runner.os }}-KeySetCache-
+
       - name: Build and test compiler (dataflow)
         uses: addnab/docker-run-action@v3
         with:
@@ -106,7 +115,7 @@ jobs:
           image: ${{ env.DOCKER_IMAGE_TEST_DF }}
           username: ${{ secrets.GHCR_LOGIN }}
           password: ${{ secrets.GHCR_PASSWORD }}
-          options: -v ${{ github.workspace }}/compiler:/compiler
+          options: -v ${{ github.workspace }}/compiler:/compiler -v ${{ github.workspace }}/KeySetCache:/tmp/KeySetCache
           shell: bash
           run: |
             set -e