Cache Go dependencies and build outputs in ci-go workflow (#75)

To improve workflow execution time for the `ci-go` workflow [1] the
`actions/cache` [2] GitHub action is now used to cache Go dependencies
and build outputs. Also see the detailed documentation about
"Caching dependencies to speed up workflows" [3] to learn more about the
way `action/cache` works.

[1]: https://github.com/svengreb/tmpl-go/blob/c099c6ee246eb402b63f7a605ca647c481a02eab/.github/workflows/ci-go.yaml
[2]: https://github.com/actions/cache
[3]: https://docs.github.com/en/actions/advanced-guides/caching-dependencies-to-speed-up-workflows

Resolves GH-74

.github/workflows/ci-go.yaml

@@ -56,3 +56,23 @@ jobs:
           go-version: "1.17.x"
       - name: Run tests with coverage and race detector
         run: go test -cover -race -v ./...
+
+  post-tasks:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Print metadata and context information
+        run: |
+          echo "Git SHA: $GITHUB_SHA"
+          echo "Git Ref: $GITHUB_REF"
+          echo "Workflow Actor: $GITHUB_ACTOR"
+      - name: Checkout repository
+        uses: actions/checkout@v2
+      - name: Cache Go dependencies and build outputs
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.cache/go-build
+            ~/go/pkg/mod
+          key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
+          restore-keys: |
+            ${{ runner.os }}-go-