Replace message for existing sha in `uenv image pull` #183

Job	Run time
build-and-test (debug, none)	4m 10s
build-and-test (debug, address)	4m 53s
build-and-test (debug, leak)	4m 18s
build-and-test (debug, undefined)	5m 35s
build-and-test (debug, thread)	4m 42s
build-and-test (release, none)	5m 1s
build-and-test (release, leak)	5m 9s
build-and-test (release, undefined)	13m 46s
build-and-test (release, thread)	5m 34s
	53m 8s

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