test: add integration build tag (#2831)

This test verify the integration of the librarian tool with external
systems, GitHub and Artifact Registry. It has expectation setups in env
vars and auth to properly work.
With an `integration` build tag, all tests in system_test.go will be
skipped when running `go test ./...` locally. Which is same behavior for
most developers who does not have relevant env vars setup.

Also remove the temp skip for TestFindLatestImage because it is no
longer triggered with standard `go test ./...`. Added a note to make
debugging easier.

Fixes #2720

Author: zhumin8

system_test.go

@@ -12,6 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+//go:build integration
+
 package librarian
 
 import (
@@ -446,7 +448,7 @@ func TestCreateRelease(t *testing.T) {
 }
 
 func TestFindLatestImage(t *testing.T) {
-	t.Skip("Temporarily skipping until we can run it reliably. See https://github.com/googleapis/librarian/issues/2720")
+	t.Logf("Note: This test requires authentication with the Artifact Registry in project 'cloud-sdk-librarian-prod'.")
 
 	// If we are able to configure system tests on GitHub actions, then update this
 	// guard clause.