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](https://github.com/actions/cache/compare/v2...v2.1.5)

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

.github/workflows/test.yml

@@ -25,7 +25,7 @@ jobs:
         go-version: ${{ matrix.go-version }}
 
     - name: Cache dependencies
-      uses: actions/cache@v2
+      uses: actions/cache@v2.1.5
       with:
         path: vendor/
         key: vendor-${{ matrix.go-version }}-${{ hashFiles('go.mod', 'go.sum') }}