Add caching to CI jobs (#13712)

- Seems to be improving times for shorter jobs at least.

- Remove Go 1.16.x tests for IAM and replication

.github/workflows/go.yml

@@ -24,6 +24,14 @@ jobs:
       - uses: actions/setup-go@v2
         with:
           go-version: ${{ matrix.go-version }}
+      - uses: actions/cache@v2
+        with:
+          path: |
+            ~/.cache/go-build
+            ~/go/pkg/mod
+          key: ${{ runner.os }}-${{ matrix.go-version }}-go-${{ hashFiles('**/go.sum') }}
+          restore-keys: |
+            ${{ runner.os }}-${{ matrix.go-version }}-go-
       - name: Build on ${{ matrix.os }}
         if: matrix.os == 'ubuntu-latest'
         env: