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

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] <support@github.com>

.github/workflows/build.yml

@@ -106,7 +106,7 @@ jobs:
       cache-version: v1
     steps:
       - uses: actions/checkout@v2.3.4
-      - uses: actions/cache@v2.1.5
+      - uses: actions/cache@v2.1.6
         with:
           path: ${{ env.package-dir }}
           key: ${{ runner.os }}-cygwin-${{ env.cache-version }}