e2e: skip flaky tests in nightly runs

nightlies call `make test-e2e` so skip the
flaky tests here. Presubmit lanes (= running pre-merge)
should still run all tests

Signed-off-by: Francesco Romani <fromani@redhat.com>

Makefile

@@ -101,7 +101,7 @@ $(GOBINDATA_BIN):
 
 test-e2e: $(BINDATA)
 	for d in core basic reboots reboots/sno deferred; do \
-	  KUBERNETES_CONFIG="$(KUBECONFIG)" $(GO) test -v -timeout 40m ./test/e2e/$$d -ginkgo.v -ginkgo.no-color -ginkgo.fail-fast || exit; \
+	  KUBERNETES_CONFIG="$(KUBECONFIG)" $(GO) test -v -timeout 40m ./test/e2e/$$d -ginkgo.v -ginkgo.no-color -ginkgo.fail-fast -ginkgo.label-filter=!flaky || exit; \
 	done
 
 .PHONY: test-e2e-local