Upgrade: bump actions/cache from 2 to 2.1.5

Bumps [actions/cache](https://github.com/actions/cache) from 2 to 2.1.5.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](actions/cache@v2...v2.1.5)

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/ci.yml

@@ -30,7 +30,7 @@ jobs:
       - name: Get cached node modules
         id: yarn-cache-dir-path
         run: echo "::set-output name=dir::$(yarn cache dir)"
-      - uses: actions/cache@v2
+      - uses: actions/cache@v2.1.5
         id: yarn-cache
         with:
           path: ${{ steps.yarn-cache-dir-path.outputs.dir }}