Change shipper to not overwrite all external labels #13628

Job	Run time
Go build for different platforms	8m 23s
Thanos end-to-end tests (8, 0)	4m 26s
Linters (Static Analysis) for Go	1m 7s
Thanos end-to-end tests (8, 1)	3m 46s
Thanos end-to-end tests (8, 2)	3m 21s
Thanos end-to-end tests (8, 3)	3m 51s
Thanos end-to-end tests (8, 4)	6m 36s
Thanos end-to-end tests (8, 5)	4m 0s
Thanos end-to-end tests (8, 6)	3m 59s
Thanos end-to-end tests (8, 7)	3m 36s
	43m 5s

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