Fix a flaky failure in VM Agent e2e test "testExternalNode" (#6154) (… #28804

Job	Run time
Check whether tests need to be run based on diff	15s
build	9m 17s
build-antrea-mc-controller	2m 59s
build-scale	1m 49s
build-ubi	14m 53s
build-windows	15m 53s
build-flow-aggregator	2m 11s
	47m 17s

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