build(deps): bump actions/cache from 2.1.5 to 2.1.6 (#6073)

Bumps [actions/cache](https://github.com/actions/cache) from 2.1.5 to 2.1.6.
- [Release notes](https://github.com/actions/cache/releases)
- [Commits](https://github.com/actions/cache/compare/v2.1.5...v2.1.6)

Signed-off-by: dependabot[bot] <[email protected]>

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

.github/workflows/tests.yaml

@@ -62,7 +62,7 @@ jobs:
         echo "::set-output name=dir::$(pip cache dir)"
     - name: set pip cache
       id: pip-cache
-      uses: actions/[email protected].5
+      uses: actions/[email protected].6
       with:
         path: ${{ steps.pip-cache-dir.outputs.dir }}
         key: ${{ runner.os }}-pip-${{ hashFiles('setup.py') }}