Bump actions/cache from 1 to 2.1.6 (#5252)

Bumps [actions/cache](https://github.com/actions/cache) from 1 to 2.1.6.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](actions/cache@v1...v2.1.6)

---
updated-dependencies:
- dependency-name: actions/cache
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

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

.github/workflows/ci-testing.yml

@@ -39,7 +39,7 @@ jobs:
           python -c "from pip._internal.locations import USER_CACHE_DIR; print('::set-output name=dir::' + USER_CACHE_DIR)"
 
       - name: Cache pip
-        uses: actions/cache@v1
+        uses: actions/cache@v2.1.6
         with:
           path: ${{ steps.pip-cache.outputs.dir }}
           key: ${{ runner.os }}-${{ matrix.python-version }}-pip-${{ hashFiles('requirements.txt') }}